The ideal thing would be to charge the low/weak cell directly, but unless you have a charger that does that, prob not going to happen. Why is one cell at 4.2v? Did you try and charge them and this is what resulted? Were they charged and have been sitting for a while and this is now what you have?

Since the voltage is less than 6v, most chargers won't do anything with a lipo at this low voltage. You need to discharge the high cell and then try and charge it as a NiMh at a low rate like 200 - 300ma until total voltage is back up over 6v. Then stop and try balance charging and hope balancer will even the cells out. If you try charging in as a NiMh now, you'll overcharge the 4.2v cells and harm them.

Know anybody with a chzrger that charges thru the balance wires?