Ammonia Spike In Cycled Tank

have you done anything to the filter recently prior to the clean you mentioned , it sounds like for some reason you have lost your ammonia eating bacteria, i would seem that you are now doing a fish in cycle.
4 ppm is far to high if it is indeed 4.0 ppm you need to take as much water from the tank as you can and add  new water , make sure the new water is at the same temperature as the water your removing, then retest again, you need to get the ammonia level back to .25ppm or even better 0ppm and keep repeating this process every time you get a reading higher than these levels.

As for the moss balls i am fairly sure that they could not raise the ammonia levels on themselves (althow iam willing to stand corrected on this ) but i suppose it is possible there was some sort of contamination on them that killed your bacteria and as for removing all your plants it may be a idea to pop them back in or at least a few of them as i understand it plants use ammonia so they can help you to keep the ammonia levels in check while you rebuild your bacteria colony.