Unless it's a long and hilly commute, don't even take weight into consideration. A 4 mile commute will probably take no more than 15-20 minutes depending on terrain, traffic lights and such. A lighter bike will only shave mere seconds off your time, and is probably not worth the extra money...unless you have to haul it up and down stairs or something.

Quality components. Good build quality. You want something dependable. You don't want to be on your way to work and have something come loose and/or break.
Get something that can accommodate fenders and a rack.
If you're going to be using panniers, make sure the chain stays are long enough that you don't have to deal with heal-strike.
Make sure it's a good fit for you, and that you're comfortable.

I have a 20" 2012 Trek FX 7.5. I'm 6'2", 265lb, size 15 shoes (a major consideration when it comes to preventing heal-strike). It works perfect for me. I have it outfitted with SKS Chromoplastic fenders and a Topeak Explorer MTX rack/trunk/pannier combo.

I bought the bike used, and all together, I think I have about $500 into the bike and the components mentioned. Probably more like $800 if you include the grips, lights, shoes, pedals, bottle cages, saddle, etc. For me, it's all about making it safe and comfortable. My commute is usually about 25 miles r/t. I like to take the scenic and hilly route home, so I prefer something that I can comfortably climb hills with.

Or you could just go the route of riding a mountain bike and wearing a backpack. As long as it gets you to work and back in one piece.