I did some quick searches and didn't come up with any good comparisons of differences between Samba3 and Samba4. I did notice that both packages are available with 19.07.0.
I currently use Samba3 as a way to backup Windows clients. On the clients I have Cygwin installed and use a rsync script to backup files to a USB hard drive hooked to my router. Of course using Samba allows Windows file explorer access both ways.
For my limited needs, is there any reason to look at Samba4? I'd also appreciate reading about what Samba4 brings to the table in general.
We are still in transition period, so my first advice is to keep things as they are if it works they way you want it.
BUT samba3 is EOL for several years now (2016), aka does only get security updates from dedicated maintainers (backports). So most of the "serious" world has moved to samba4 already, yet samba3 has a prolonged life on embedded devices. Mainly because samba3 can be stripped down to the basic fileserver component aka 2-3 MB, while samba4 is much harder to strip down and so we end-up with 7-30MB.
The next mayor difference is that samba3 only supports up-to SMBv2.1, while samba4 supports SMBv3.1.1.
The next difference is that samba4 and OpenWrt has VFS module support. Aka things like exposing shadowcopy directly to windows or allowing to act as a MacOS Timemachine target.
The next more exotic option is that our samba4 package can also be build with experimental full AD-DC support.
Than recently we added smbd, which in contrast is a tiny (soon tobe 200KB) basic SMBv3.1.1 compatible fileserver. Its more resource friendly, yet rather new so not as tested as samba3/4, but is actually in a pretty use-able state.
Over time i expect smbd to fully replace samba3/4 on embedded/low powered devices and samba4 is only used if a extra feature is needed (VFS, Domains, AD-DC...)