I've used backpacks and panniers, both with pannier bags and a trunk bag. I bought a used Burley d'Lite to carry around big loads.

Panniers are nice if you're going some place and not really walking anywhere. Even if you have a shoulder strap, and even if you have fenders, they can get nasty if the weather is less than great. You don't want to be carrying these around much. They also take a little bit to get on and off.

Whatever you put in your panniers will also get the bejeesus shaken out of it. Don't open the beer right when you get where you're going, not that you're that kind of teacher.

It is really nice not to have stuff on your back though. If your commuter was longer than 6 or 7 miles I'm sure you'd appreciate the panniers.

Backpacks don't have to be nasty. I've seen Deuter and Marmot packs with mesh liners, not that those are the only brands. My bag has a hip strap. I cinch that down, leave the shoulder straps slack, and viola - the bag rests on my hips and not my shoulders.

If I were you I'd buy one of those fancy schmancy "air flow" backpacks like the Deuter Futura 28 and buy a separate laptop sleeve. At 4 miles a pannier seems like overkill given the additional risk to your equipment from the vibration.