Tom,
Four cells in series should measure 14.4V, each individual cell should provide 3.6V or there abouts depending on S.O.C… I’d suggest your measuring individual cells based on your numbers, noted above.
Best practice would be to balance charge the cells, this is a great time to add a balance charge pigtail… It would be prudent to replace defective cells with like OEM cells, the caveat herein is the condition of the balance of the cells in the pack. The sum of the battery is only as good as the condition of the lowest performing cell.
Michael