Maybe discharge the pack some either running it or with charger and reset to charge at 0.5 amps. It will take a long time and probably have to reset charger since it shuts off after 2 hrs but this should bring it around.
IF not, then you might have other problems such as a bad balance tab possibley ?
Hope this helps ?
I retired from RC, now life is all about guns and long range shooting.
I had a Turnigy 4s 5000mAh pack do even worse. But after charging and discharging several times (balance) it came back around pretty close. The one weak sell now charges up to 4.19. Close enough for me. But I would still keep an eye on it while your charging it. Don't want no lipo fires