You need to have your existing v3 users specify version 3 in their mounts. Do this before you enable v4 on the cluster and you should see no change in behavior after the change. Don’t do this however, and you’ll see exactly what you just mentioned which is that clients will negotiate the highest mutually supported version the next time they reboot (or unmount/remount). So the problems may not be readily apparent. Version 4 is a different beast, that is stateful, supports ACLs, has different failover behavior, and much more. It’s not the case with NFS that newer is always better.
Hope this helps,
Principal SE, Datadobi