We use Multiplay for ours, but we didn't use the "FTB" option, we just use a vanilla install and stick the mods in ourselves. We have the 1 Gig option (and it's also a 4 person server, although there are currently only 2 of us playing there). Sometimes if there's a lot going on the server restarts itself, it was fine until we started using FTB (we did have a 1.2.5 tekkit install a while back with forestry added that worked without the restarts). I reckon it needs more RAM really, but we are currently happy with the way it is right now, will pay more if we start getting problems.