For a four mile commute, which I have had in the past, I don't bring any tools at all. It's too short a trip to bother and I generally don't run so late that adding ten or twenty minutes to my trip by walking is going to make me miss an appointment. YMMV.

For short town trips, like your commute, I like having a metal basket on the front of my bike. I just throw whatever-sized bag I'm using that day into the basket and hold it down with an elastic cord and I'm good to go. It's quick, easy and versatile.