I can't answer the question myself yet. I was able to acquire an 8GB Pi 4B at a somewhat reasonable price, tho I had to buy a kit. I installed the latest 32 bit Raspberry Pi OS. I got DepthAI running (with a bit of difficulty; you might search for my post on the subject in Discord). I've used the system as a laptop (keyboard and monitor). I don't have plans to use it that way, so I did only limited testing (rob_preview.py), and it seemed to run roughly the same speed as my Mac. I next tried rgb_preview.py using VNC over wifi. Not good; maybe 1-2 FPS. It think the answer to your question is that it depends on what you plan to do with the system and how.
I've read that (a) the 32 bit OS can only address 4GB, and (b) the 64 bit OS works well at least for a headless environment and is faster, and can use all 8GB. I'm hoping to do some experiments to determine how much memory gets consumed by various pieces of the system and will report. Possibly here and in Discord.