Congrats,

4.5 miles is a good beginning distance. My commute to school is about that length. It takes me much longer to get to work.

As far a vehicles whizzing by your elbow, do you take the lane? When I started commuting I rode too close to the curb and cars passed me in the same lane I was traveling in thus they were pretty close to me when they passed. If you take the lane by getting far enought off the curb that the cars have to go into the next lane to pass you, you'll get more space and the ride will be more pleasant. It seems counter-intuitive at first, but it works, and in most states it is legal.

Congratulations again, it's better on a bike.
--AK