Yes. @anon50890781 already gave you pointer to respective issue, I am just confirming that there is going to be one.
Backups from 3.x are mostly incompatible and it is highly suggested to not use 3.x backup on TOS 4.x. Only supported migration is clean reflash with new configuration or direct migration from TOS 3.x.
Snapshots are going to be preserved in migration and there are ways on how to preserved them even if you do clean installation instead of migration. (schnapps import -f medkit-4.0.tar.gz && schnapps rollback factory which updates factory image to given medkit and does factory reset).
Depends on what you are using. Primary changes are probably drop of Cups, Majordomo and api tokens. There is more of course but most of the changes are given by about two years of upstream changes. It really depends on what you are using on router.
After automatic update, no. It should hopefully be smooth update like any other. But once again it depends on your router usage. Some services might have to be reconfigured and update might have more blocker than usual.