You can use active balancers and just keep using it. My 12 volt lifepo4 also has one cell that has always been low, when charging it reads 3.40 volts while the other 3 read 3.60 volts. I wasn't going to rebuild the pack with new cells. Last year I bought some active balancers, and the battery has stayed in perfect balance ever since, I use it every day.
Cost is about 100 dollars for a 4s active balancer module. You just connect them to the 4s balance connectors. They are always balancing in the background. They transfer amps from the high cell to the low cells, and can handle up to 10 amps per cell, you can fast charge your battery pack.
electriccarpartscompany sells them. I think it will be cheaper then rebuilding your pack. Even though your battery pack is 6 years old, it looks like it still got plenty of life left in it.
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