You started with 4 platys and let the ammonia level climb to 5? That is very cruel. You must do water changes right away to get that level down. You only let those levels climb if you are doing a fishless cycle, not one with fish. Ammonia will kill them off, no latter how hardy they are, plus it will leave them very weak when the cycle is over. Do water changes to get that level down to zero.
Doing a cycle with fish takes much longer because you can't let the levels climb up at all.