I've always heard if carpet is wet for over three days mold can happen. The thing that looks off to me is there is no rust on the tac strip or carpet backing, and the wood does not show any signs of rot. That is quite a bit of mold for 4 days, but in a humid climate, I guess it could happen. Man, this is a tough one for you. Was that the only spot with mold? If so localize it and treat it, then get some air movers down and dry that puppy out. If not localized, then you're dealing with the spreading the spores. Ugh, Good luck!