Beware - I updated my S20+ 5G (SM-G986B/DS) to the new One UI 4.0 this morning, OTA. The phone is basically unstable and nearly unusable at this point. Half the apps don't open (including the Samsung Member's app - I'm sending this from a laptop instead). Turning on airplane model causes a reboot. WhatsApp stopped working (wont send or receive messages), and did a half-dozen other apps (including Instagram).
As I see it, there's two options: factory reset and attempt a firmware downgrade, or wait for Samsung to fix this unstable release. Thankfully I still have my S10 hanging around. Might need to use that for a day or two until these bugs are worked out.
But my advice for all S20+ users: Do not update until a patch is released. I'm a longtime Samsung fan who was planning to get the Z fold in April - but this has me very nervous. Releasing an untested and buggy update OTA and rendering devices useless is reckless.