It sounds to me like the OP doesn't have a bike. If that's correct, I'd suggest buying a bike from a good local bike shop (LBS). A mountain bike (can you get slick tires in the price?) or hybrid should fit into the budget, along with lights, patch kit or spare tube, pump, and perhaps a rack and saddle bags (panniers).

I don't doubt OP's mechanical aptitude, but it'd be worthwhile to have a reliable bike for the first 6-18 months to get into the habit of commuting before mechanical issues start rearing their ugly heads. Four miles from home and work is no place to start messing about with derailer adjustment, or brake adjustment, or any of the other little things that can go wrong. A good LBS will make sure that's what they sell, whether new or (more rarely) used.