Watch4 Classic, LTE version running on AT&T here.
Finally got the software update to download onto my Z Flip3, then had major trouble getting the update to transfer to the Watch4. Would get notice on my phone that update was copying to the watch and a notice on the watch that it was copying update. Then all would stop and I would get a notice on my phone to the effect of: "Could not copy updte to watch. Turn on wifi on watch and try again."
Very strange -- checked the watch and it had wif turned on already. Went back through the whole cycle -- download, try to copy, copy fail, etc. -- about 5 times over the course of two hours.
Finally turned off the watch -- complete power off -- and let it sit for a minute or so. Restarted the watch, waited for it to reconnect. checked agin for update on the phone and all went through successfully.
Now running R895USQU1BUI5, August 1, 2021 security patch version. Size was 158.35 MB.