Have you got a water testing kit, Nemesea?
That would be a good place to start - check out exactly what it happening with the water conditions before you start treating your axolotls. Salt baths can be nippy for axolotls, and the constant changes in water and tubs could be distressing.
Salt baths are typically used for fungus, and tea/almond baths for soothing skin conditions, and salt baths in particular should not be used as a 'catch all' treatment.
I would suggest keeping your axolotls in clean dechlorinated water in the fridge for the time being until you can test your water and work out what is causing the problem. Doing a very large water change may have killed your
bacteria cycle and caused a spike of ammonia, or it might just have been too big a temperature change. How big is your tank? How hot did it get?
There are some good tips on
cooling here, and tons of other great threads on the forum.
Hopefully your 4 axies will fully recover, give them some fresh dechlorinated water every day and do some testing on your tank, see what the matter may be. Some local fish shops may test your water for you if you don't have a kit, by the way
If we do get this second run of hot weather they keep promising us, you'll want to be prepared!
mew x