Samba 4.x is an intimidating piece of software. If it can perfrom the same function and use the same config, I'm willing to try it.
Without log messages or process table info, it's hard to advise any further. Generally speaking, Samba4 can do everything that Samba 3.6 does. If your server is a domain member or standalone, it should be relatively straightforward.
I migrated my Samba 3.6 servers to Samba4 when the badlock vulnerability was announced. My servers are AD domain members, so the migration was fairly simple. Without knowing which role (domain controller, domain member, standalone) your Samba server is providing, it's difficult to comment on the complexity of migrating your server to Samba4.
Consider spinning up a VM that is similar to your current server, including the same Samba 3.6 packages and your current smb.conf. See if you can replicate the problem your users are experiencing. Try upgrading the VM to Samba4 to see if the enabled features in your smb.conf file work with Samba4. Running testparm should tell you if it works (or not). When I performed my upgrade, I first had to discover which Samba 3.6.x packages were installed, then I had to remove those packages before installing the Samba4 packages. Backup your current smb.conf file prior to removing your Samba 3.6 packages. Check here for info on updating Samba: https://wiki.samba.org/index.php/Updating_Samba
Andrew