This is a good point since it can't be determined just by looking at the files (altough opening the editor is surefire way to know). One difference would be that 3.x projects have .import folder whereas 4.x projects have .godot folder in them. However, including these with the project is not required since they will be regenerated upon opening the project if they are not present (they include cache files).
Personally I can work on projects from both versions, but I would like to know what other godotters are up to. If I'm going to do day one, I'd like to hear some preferences. My take is that while 4.x has many improvements, web export support is a bit lacking atm. Which is, frankly put, a big downside for jam projects because download only games tend to get smaller amount of plays and ratings. On the other hand, this jam's concept is quite different so maybe it's Godot 4's time to shine?