I've always heard it was best to have a matched set of batteries to avoid problems in the long run. I'd replace all of them. Take a picture with your phone of how they're wired before you begin taking anything loose. They should be wired as two pairs in series. The positive on one battery goes to the negative on the second one. You will get 12v from that set. Do the same for the other set. Then both sets are wired in parallel so the 12v from the two pairs stays the same. That doubles the amperage available. Take a picture and then make a drawing of what's in the picture. Label the wires. Label makers aren't real expensive.