I'm doing to do a similar thing sometime before xmas but I'm planning on having internal storage. Basically the server will serve media to my PS3 and a popcorn hour, download and make backups.
What I've decided on is getting an Intel Atom mobo, a cheap small case with space for 3x 3.5" drives and 1Gb of laptop RAM. All of that shouldn't come to much more than £130, add some storage and it should be good to go. I'll probably run windows home server though but that's an expense you won't have.
I did look at pre-built options like the Eee Box but the selection is very limited at the moment and none of them were exactly what I was looking for, in the near future there may be better options available but I'm running out of space fast and can't really wait.
As for being able to get the data to 4 devices at once, I'm not sure, I've not looked into it as I'd only be using one device to get the media at once.