I also got a 4 on AP Physics B. My teacher wasn't very good though, we didn't cover all the material we were supposed to. She told us if we wanted to get a 5 we would have to self-study the rest of the material (we skipped waves, I'm not sure what else).
As long as you are using good materials to study for the mcat you should be fine only having Physics B. I ended taking Physics I and II in college. I took them because I knew they would be easy A's and I also thought they would help with the MCAT. It made it so that I don't have to study Physics much for the MCAT but I don't think my score will be any different than if I had not taken them in college.