I see a few options
1. The battery balancing circuit had a brain fart and drained #4. Once it is balanced, you'll be fine.
2. A cell in #4 failed open. Once the pack balances at the high SOC, things will look good, but total pack capacity will be reduced and when used, #4 will hit the voltage floor first. However, the pack will charge back up to the high SOC.
3. There is a resistive fault in #4. Any time spent off the charger will result in increased imbalance.
1. The battery balancing circuit had a brain fart and drained #4. Once it is balanced, you'll be fine.
2. A cell in #4 failed open. Once the pack balances at the high SOC, things will look good, but total pack capacity will be reduced and when used, #4 will hit the voltage floor first. However, the pack will charge back up to the high SOC.
3. There is a resistive fault in #4. Any time spent off the charger will result in increased imbalance.