For a 4 mile commute, most any bike will do. Around 4 miles I do start thinking more hand positions would be nice. The main thing is to get a bike that fits, and you enjoy riding.

Mountain bikes with slicks make fine commuters. I have an old rigid bike that was a great commuter. I also have a hardtail with slicks that is nice for quick commutes.

Personally I like the mid 80's to 90's rigid MTB's as commuters. They had beefy frames, stable geometry, many had eyelets for both racks and fenders, and can usually be found for not much money. The only drawback was that they could be a little on the heavy side