well you really haven't said much about what you're actually doing here, but anythign with only four users can't be that intensive... I'd imagine that if you do want a box with "server" formally written across the side, then just about the lowest spec server dell do would be fine really. these days the average spec outperforms what most people need. One thing to mention is that you probably don't *need* to use RedHat at all, id' suggest you think about CentOS instead if you'd like *exactly* the same operating system (apart from logo's) and save money to put onto a better machine if you want.
You would probably want to look at doing raid 1 for data integrity on the box and all that though, which would increase the hardware requirements, but only nominally so. i'd still just stick with a standard sata drive now though.
btw, do you actually *need* your own server? what if you used an online hosting company to host it? totally depends what it actually is, but there are plenty of *very* well priced linux server hosting companies which can take all the hardware concerns away totally. jsut buy a bog standard PC and off you go on the client side.