It’s been in the database since April 2022, so it’s definitely in OH4.1.2.
Given the size of your system, I’d think days, not months. While you do need to review the Getting Started docs, you aren’t entirely learning from scratch. Once you’ve made a few things and items, it’ll be pretty smooth sailing adding everything else.
You can avoid the Z-Wave hassles by using the Remote Binding (as suggested earlier) to access your OH2 system from OH4. Then when you’re ready, just move the Z-Wave controller to OH4.
Before you get too invested in Z-Wave, it’s definitely worth seeing what’s available for Zigbee in your area. I’ve generally found them to be significantly less expensive and easier to find than Z-Wave sensors. Apparently, the new Sonoff temp/humidity sensors are pretty good.