Almost anything. Mine has 1GB of ram and it can do vpn, nginx proxy and other stuff just fine. Use dietpi.

MySQL, pihole, nextcloud and secure pastebin. Bunch of dotnet and python apps. Also running a notification service.

Ok here it goes...

Many on this sub will start an echo chamber. They will tell you how you should never host e-mail. Never! And how this plan, and that host, is so much cheaper. It's not worth the headache. Oh the headaches. Bla bla bla. They think because they don't want to do it, no one else should.

If you want to learn how to host your own e-mail. Do it! Just don't plan on using that e-mail for anything mission critical. At least not in the beginning.

Oh and your Pi4 is more than enough to do what you are trying to.