The very fact the snake has been in a poor set up from the get go, before you took it into the care, the more important it is to have it brought to a vet right away, not 4 days later. Who knows how long that wound has been on the snake before you acquired it. This sort of situation requires immediate treatment by a qualified herp vet.
It doesn't take hours of research online to know large open wounds become infected, especially when flies and other insects have full access. Though topical antibiotics do help kill germs and bacteria they are not designed to work internally. Septic wounds poison the blood and require injected antibiotics to be administered to a vet. The wound will need to be cleaned out and all necrotic flesh removed, something that should be done by a vet. Flushing with clean water will help remove debris etc but won't be very comfortable for the snake. In fact it could be quite painful.
I do applaud your efforts to give this snake better care but there is very little one can do at home, and as Faust points out, without seeing the injury itself, its virtually impossible to even guess at what is going on with the snake.
Perhaps my earlier post was a bit strong, but I see countless posts of people 'rescuing' sick and injured animals who have little to no clue what they are doing and they come here for help. Some even outright say they won't bring it to a vet. I am glad you are making the effort to get this snake in to see a vet, though I do worry Tuesday may be too late. I hope it isn't, as I would love to hear the snake pulled through and is well on his way to a long healthy life in your care.
The fact the animal is eating and pooping regularly doesn't mean its not in pain. Many animals, especially reptiles, 'hide' sickness and injury to avoid looking 'weak' to predators. Often by the time you realize something is seriously wrong, its too late to do anything about it.
Can you take some pictures of this injury and post them? Having a visual of the injury will be helpful to give other guidance besides 'take it to a vet', to hopefully increase the chances the snake survives till the vet appointment you have for Tuesday.
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PHLdyPayne