Yeah, road bikes work fine if you prefer a nimble ride. I personally hate road bikes for the most part, because I hate thin tires and prefer the ability to run fatties and fenders, racks if I choose. That said, most people I see commuting in the DC area are on road bikes....go figure .

Many bikes can be great commuters -- just get some good tires, learn to change a flat, and find a good saddle.

For a 4 mile commute you're a good candidate for a messenger bag, since sweat won't be profuse and comfort will likely be good. I commute only 6 miles (12 round trip) to work now, but with my old job it was more than 2X that. For the short commute I use a XL timbuk2 messenger bag that I really like. For the old commute I had front and rear racks and panniers -- which worked very well. I'm happy with either really. I just like to ride my bike .

I've found a really good way to secure a timbuk2 bag instead of using the cross (stabilizer) strap -- if anyone is curious, let me know.