Freshwater dips do not effectively treat a fish for anything. They CAN be used to provide temporary relief to a fish heavily infested by external parasites in conjunction with another treatment, but they will not eliminate anything by themselves.
Brook is not limited to clowns, it is just seen most often in clowns. Brook is so virulent it usually kills faster than weeks in aquaria, unless the brook is IN the tank and they were exposed to it upon addition to the tank; then the timing is about right.
Healthy fish can sometimes fight off brook for a very long time, so we can't eliminate brook as a possibility. However, brook is so deadly that any hatchery with brook in their system will lose all their fish over a very short period of time -- brook in CB fish is very, very rare. If the fish came from a fish store, went through a wholesaler or at any point were in a shared system, however, all bets are off.
Severe internal parasites is a possibility, but again, this is extremely rare in CB fish, and to reach that stage the fish will have had to have stopped eating for some time before death.
Adding 4 at once raises the possibility that they killed each other in a normal battle for sexual dominance and territory; the "winner(s)" have been damaged and succumbed to infection. This is most likely if all 4 were the same size; otherwise a pecking order is more obvious and fighting is reduced.
I would remove all fish to QT for observation and hypo, and allow the main tank to be fishless for a minimum of 6 weeks. If the fish in QT show symptoms you can treat them and then get a better idea of what the illness might have been, but without a list of symptoms and more detailed info on behavior before dying, it's just a guessing game.
No matter what else you do, I would not add any more fish to the tank for now. Carefully observe your current fish for symptoms -- and diligently track your tank parameters including O2 and pH -- for at least a couple of months. As long as fish remain in the tank, though, anything communicable will be a resident in the tank and might affect any new additions. So, the QT routine and leaving the tank fallow is the safest option.