you _could_ commute on the nice road bike, but i think picking up a cheap MTB or touring bike would be a lot better. less than $250 with rack and fenders should be no problem...

i personally like to fully change clothes for any commute over about 2 miles so that i can ride hard (for the last 2.5 years i have had a 7mile commute which is just enough to do some decent training) and not worry about sweating all over my work clothes. i wear full bike gear and carry my clothes rolled in a pannier and change completely from underwear to socks to shoes in the company bathroom (fortunately ours are very clean as have been bathrooms at most companies i have worked for). we don't have showers here. ok, here isn't as hot as Dallas (i lived 18 years there) but if you ride slowly the last few minutes to cool down and then do some kind of a cool down walk or whatever, then go to the bathroom and wash your face, change your clothes and then wait a few minutes you will not be too sweaty. i used to do it in Houston TX. i know some guy uses paper towels or hand-wipes or whatever, but i find if i shower in the morning, ride to work, change and cool off i don't smell or anything...

panniers are better than backpack b/c not so hot (sweaty back).

4 miles is totally doable year round! try and ride 2-3 days a week and see how it goes. especially in winter the training is GREAT. i am already fit in th spring when everyone else is out of shape!

good luck!