Changing battery modules requires special factory training and special equipment. It's also expensive. And there's no guarantee that the remaining modules will not fail at some time in the future.
A123 had 3 manufacturing lines, one of which had prismatic cell alignment problems. This was discovered after the Consumer Reports test car completely shutdown due to multiple cell failures.
IMHO, replacing a couple of bad modules is worth the risk. But four going bad is a big red flag. You should think about getting a new battery.
Karma - Shadow/Canyon #1292