If you’re talking about RPi4 only then you can restrict it and the answer is rather simple: pay $10 for one GB or just $5 ?.
Hopefully you’re referring to ZRAM not tmpfs.
And zram won’t need as much RAM for the same purpose and I’d say you’re back to 2GB for the sweet spot.
Either way, the true discussion is not on which RPi4 model to buy but if to buy at all or stay with your RPi2/3 w/ 1GB. Which would be an even sweeter spot 
albeit not compatible with the thread title.
For one, your JVM RAM usage is awfully high, way above average. You should get well below 1GB unless you run really exotic stuff or stuff to have mem leaks.
[ although that’s something we should have another closer look at - I have a strong feeling that the changes in 2.5M2(?) make up for a serious increase in RAM demand. My own brutto RAM demand has always (up to and incl. 2.5M1) stagnated at ~700MB now it jumped to >900M
For two, the relevant column is the “RES” column which is what you’re effectively keeping in RAM. So there’s well headroom to even run zram and do it all on a 1GB machine.