"I’ll need to run my website and e-mails etc via the server as well"
Whch isn't a very good idea - at least the website part isn't. I assume you do mean a website, and not an intranet? You say there are only going to be four people on this network, so I imagine it's not an intranet.
In that case it's a bad idea to run an office network on a server that's accessible from the internet - it would necessitate all kinds of security measures that might be a bit too involved (and expensive) at this stage. I suggest that you simply run your site with a web-host, it will be cheap, and updating is a piece of cake.
You would then have no need for a dedicated mailserver - just run as many mailboxes as you like on the hosting account.
You'll certainly need an automatic backup facility, but how you handle this depends on two things - whether you want to backup from individual workstations, from the server, or from both, and on how much data is going to be involved. If you're going to run the database on the server then you'll want to back that up separately anyway. Other than that, is this machine simply going to be a fileserver, with applications like Microsoft office running independently on each workstation?
The reason I'm asking these questions is that often it's not at all necessary to run Small business Server on a four-machine network, you could simply run Windows XP or Vista on each machine and network them on an ad-hoc basis.
Perhaps if you could come back with more information I can be more helpful.