Nope. New OS.
Backup and restore is going to be my first option. Will see this after.
The new hardware is not backwards compatible, so a complete new version of Raspbian is required even to boot (stretch -> buster). This means you can’t simply swap uSD cards today.
An upgrade is likely to need taking a backup of your OpenHAB files from the uSD card, creating a new card with a new software image, and restoring your backup of OpenHAB config to the new install. The good thing is uSD cards and RPi are relatively cheap, you can afford to have several - keep your ‘old’ system running you home whilst you test on a separate per-production device.
This means if hardware fails in the future, you have ‘cold spare’ backup hardware and software ready to go. You do take backups of your OpenHAB config don’t you? 
It is possible that the current version of Raspbian (e.g. my OpenHABian RPi3b runs Debian stretch kernel 4.19.42-v7) may be upgraded to support the new RPi4 hardware, but my experience is the old boot loader on an older uSD card still can’t work on new hardware.
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No sorry… 4MB of ram, ofcouse … Bare with me, this is more than 30 years ago… 
My Atari ST´s had 512KB as default. I had to solder the RAM modification myself.
The 1040 STé was alot easier, as it had Simm sockets. It was default 1040KB of RAM.
All my Atari computers was “upgraded” with hdd´s as well. My Atari Falcon´s was upgraded with SCSI hdd´s and Co-processors.
The ST 512, and I believe 1040, was shipped with monochrome monitors… I rewired the cable connection added a color monitors insted… I think it´d handle a 16 color monitor at that time… The Falcon could do 16mill colors by default, as far as I remember.
I got my first “real” PC sometime in the 80´s, probably around late 80´s - 1986/1987… It was a Olivetti with Motorola 80286 and dual 5.25" floppy drives and a small MFM harddrive, (added later), with a grey-tone monitor… I think this one was running one of the first version of MS DOS. (version 2.xxx or something simular). From there things went from fanatic to stupidty… At one time, I had 5 computers running at the same time. (Two running the BBS, two for music production, and one for just having fun/playing). When Atari Co. blew their status on the market, I hold on to the Atari computers for music production only for a few years, using Steinberg Pro 12/24, and “upgraded” to Steinberg Cubase. In the early 90´s I changed to be running Cubase on the PC insted, and wihtin a year, I dumped all my Atari computers. From there things went from stupidty to crazy… Everytime a new Intel processor arrived, I upgraded my PC´s, rebuilding and configurating everything myself, starting with the 80386 processor, everytime with gameplay in mind, as well as server hosting, (web, mail and DNS hosting).
Untill 2015, I had been building all my PC´s myself… In 2015 I bought my first laptop, a Lenovo T540p with Intel core I7 4. gen. processor, which I (for god know what reason) is still using… (Hmm… Mind me it´s time to upgrade I guess 
). Today, beside a few Rpi´s 3B+´s, I have an very old Qnap T110 NAS, a HP mediasmart server ex490, (modified with a quad core Intel processor and RAM, still hosting my mailserver), and a few older Lenovo and HP´s laptops with gen. 2 and 3 Intel core I5/I7 processors lying around which I bought very cheap, and my kids had been using.
I guess I´ll never learn to limit myself 
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Amstrad CPC464 was our second computer! The first was a Texas Instruments TI99-4A. Those were the days.
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I too got started on a TI99-4A. We even had the voice module (I still get chills when I think about the first time I heard Leonard Nemoy’s voice say “Welcome aboard captain” on the Star Trek game) and cassette tape for storage. I wrote many a stupid BASIC program on that thing, though I mainly used it to play games. It was light years ahead of the Atari at the time and this was pre-NES. Then when I was a freshman in high school my parents bought an Intel 386 running Windows 3.0 (circa 1989).
Of course I used Apple IIs and PC Jrs in school (I learned Pascal on a PC Jr.).
From there I was on the Intel/Windows train as my “primary driver” until fairly recently. I was exposed to Linux, Solaris, FreeBSD, Irix and just about everything else that was modern in the early 1990s at school and have dabbled with most of them on and off over the years, mostly just enough to keep up my skills. I even built a small Beowulf cluster in my dorm room for awhile, just to see how to do it.
After college I got into HTPCs and I started having a desktop and a laptop. But over time I found spending most of my time with the laptop. For a time I had a pretty powerful HTPC that I built (Windows running BeyondTV, MythTV wasn’t stable enough for me at the time). Then the RPi came out and I abandoned the HTPC for the most part and used OpenELEC on the RPi instead. Goodbye big old noisy machine and hello little device I can tape to the back of the TV. 
Then I got married and stopped playing video games so much and the need for a beefy desktop went away. I’d already been mostly using my laptop so I got rid of the desktop. All this time I’m still doing mainly Windows with dual booting and later VMs just to keep my Linux/BSD/Solaris skills current.
Then I got into home automation and everything changed. I ended up getting a desktop server machine (back to the good old black box machine) to run the servers running ESXi with all Linux VMs, and RPis all over the place. I got one of those really little Intel processor machines to run pfSense so I still have FreeBSD hanging around (and my wife has a Mac). I found myself spending 99% of my time with an ssh shell open to a Linux machine or a VNC session up to my virtual desktop on the desktop. I had to ask, why am I still using Windows?
So now my daily driver at home is a Chromebook (I have Windows at work) and I love it. I’m aware of the tracking Google does and am OK with it. It’s a fair trade, for me. YMMV. I mostly use Debian based Linux everywhere so my SuSE and Red Hat skills are definitely atrophied. Solaris is no longer maintained so I don’t do anything with it any more.
It’s been quite a journey. But I want to give my son, who is now six, the opportunity to build something with his own hands and building an all-in-one or boxy wooden laptop is perfect, especially because he will be able to see how it all works (at a high level) and experiment with interfacing the computer with the real world. I’ll see where his interests lie and expand from there as he grows. His reading isn’t yet up to where he can start playing too much on his own yet. I have time.
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If your son show some interest, let him build a simple PC, (or look while you´re doing it)… (motherboard, kabin, hdd, RAM, CPU, GFX card powersupply etc)… That will give him quite a good insight of what basicly every computers is made of. Fetching all these devices together and see it spin up afterwards, is a great reward. It dont have to be the most powerfull, just some cheap devices which is good enough to run something, (probably Windows, cause it´s good for simple games). After he has seen it a a few times… He´s ready for his first build
I got OH 2.4 working without hassle on the RPi 4. I’ll migrate over the config and use this as my main rpi for it. I’m. Curio to see if my stability problems improve.
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Thanks for the update. Great information. Please let us know how it works. Do you notice any difference? What memory version do you have?
I’ve snagged the 4gb version. My thoughts are I’ll be consolidate mysql, mosquito and OH onto the one rpi (instead of 3).
So far I’ve had
*no joy installing Zulu (install but java failed)
*no joy getting samba working (fails to part way through install)
Etc
Not smoothest start but I remain hopeful
Sounds like a good plan. I’d be very interested in the results.
Wow that is a disappointment. Still important information. I hope you will share your experiences. Maybe other forum members have solutions.
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Definitely take notes as you go. I’m certain the openHABian folks will greatly appreciate your experience.
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Check out my post on getting an odroid c2 working as I shared a bash script which is full of terminal commands which are proven to work on 64bit Ubuntu as java is a pain to get running manually and it took me a while to work out that method in the script. They may help you out but the pi4 is still very new it could be an issue that just needs a few weeks for some updates to get released.
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< evil voice>It’s alive!</evil voice>
In the end it was a combination of problems which were solved, in part, by my liver and some sleep.
It’s been a long week and I had perhaps to many gins after coming home yesterday.
- Install Buster onto RPi4
- Install Zulu as per this post
- Install OH 2.4 as per this document
- Install samba as per this document (see note below)
- Restore the backup from your other pi.
- wait for around a minute before trying to connect.
It certainly seems to start the service quite quickly. I’ve not timed it against the other but my rpi3+ takes a number of minutes before it settles down and behaves. Will see how this one works.
Some downsides which will cause rework for me -
You cannot boot from USB3 at the moment on buster. That is coming soon but not yet.
I bought a Sandisk U3 card - it has a read of c.110MB/s and write of c.70MB/s. The USB disk, a mSATA in USB3 caddy has a 300/330 rw How much that will matter in real life? Dunno. I’ll do it though for longevity.
I’ll port over the peripherals today and run this as a production system for the next week or so. Updates to follow
For the record, this is my current RPi3
You can see, memory was a problem…
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Thank you very much for this great information. Yes memory was becoming a problem. I suppose that is solved now.
As I boot my pi 3 from a ssd, I will be waiting a bit more for usb boot to become available. For anyone interested, link to documentation about missing usb boot on raspberry pi 4:
https://www.raspberrypi.org/documentation/hardware/raspberrypi/booteeprom.md
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thanks for the link Mark. Interesting reading.
I read somewhere that the boot will be here shortly so not long to wait.
Never seen any ARM based OH to use way more than 700MB.
BTW only the resident part is relevant so you don’t/didn’t have problem
(ok, the difference is being paged out so it affects SD card wearout but that happens only more or less once per OH run).
What -Xms (and -Xmx) options do you use ?
I did push it up to, IIRC, 512. I did it a while ago when there was a lot of chatter about the rules and them being cached. I have a lot of quick-run rules and thought it would help.
Another update -
This is the rpi3B+ after (service) running for a day, during the day.
Some interesting start times, and while it’s not conclusive, it’s interesting
Above is the time diff between the following lines in the log
[.core.internal.i18n.I18nProviderImpl] - Locale set to 'en_GB'.
and
[ternal.dhcp.DHCPPacketListenerServer] - DHCP request packet listener online
Between those two lines in the log file all items, things and rules are loaded. I figure it’s a good enough set of markers. If there is a better option then I’ll remeasure it.
edit:
Below is the htop of the RPi4 after 20ish minutes of running my “production” OH2 setup.
edit2:
the early morning stops and starts are thanks to insomnia. Nothing like working on home automation when you can’t sleep 
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Correction - I have the following:
xmx650 / xms400
Question - On the 4GB model, any benefit in making that higher now? Or leave it default?
Thanks for the info. I now definitely want a rp4 now. Just waiting for the USB boot.
I hear you. Where is the ‘help me sleep’ binding when you need it.