How to control cell balance FET with 5V logic?
You'll need to take care not to discharge the cells too much. With a cell voltage of 4.2, I'm going to assume that you are using some lithium chemistry batteries. If you do not have proper discharge protection, you can destroy those batteries by discharging them too deeply. When you try to charge them again, they become a serious fire hazard.
I agree with Brian that your component selection is not going to work. The N-Channel MOSFET you specify requires the gate to be driven positive relative to the source to conduct and your circuit is taking it below the source voltage when Q1 is biased on in every case except the last cell that is referenced to ground. This circuit will also take the gate of the top MOSFET outside the recommended operating range which is +/- 20V. The MOSFETs will likely be conducting when the circuit is disconnected from the logic source (or it's turned off) since the gate is tied to the drain through a resistor and the threshold voltage is 2-4 volts. The gate essentially has the cell voltage applied to it if Q1 is not conducting. Maybe this is the behavior that you want, but again, you need to have some under-voltage protection for lithium chemistry batteries. Taking them below 2.5 volts can damage them.
Here is a circuit (possibly similar to what Brian was thinking) that can operate with 3.3 to 5V logic and provides gate control that is independent of discharge rate. This circuit may be dangerous for batteries since it can discharge below 2V. 
You may be able to tune the values of R2 and R4 to get the minimum discharge voltage that you want.