In addition to the tank size issue, you could possibly have a poor gender mix... 4 mollies.... one should be male and three should be female so that the male's 'attentions' are divided enough that the females won't be stressed to the point of illness or death by constantly being pursued.
Pay extra careful attention to your water quality, making sure to keep ammonia and nitrite well below 1.0, and nitrates below 20. Also, make sure there are plenty of hiding places in the tank, adding some more plants (real or fake) if needed. Your injured fish will want to stay in hiding as it heals.