Anything with a digestive tract can get constipated. Four days isn't unheard of for a snake to poop. In terms of digestion, snakes have a balancing act between the need to get pure animal protein through the tract as quick as possible and also containing the meal long enough to absorb as much as possible. I usually get a BM about once a week if the feeding schedule is in a regular part of the season. My first advice is maybe post a pic so people on here have context and can gauge the thickness, and also sometimes the worst thing a snake owner can do is pounce on a benign problem and over "treat" it until you have a stressed out snake that NOW has health problems. If you feel compelled to intervene a warm bath can help, and at a certain point a vet may be needed because if a snake can't fix a problem with constipation itself, the chances that the average hobbyist can aren't likely.
Also, when you need to feed 4 mice at a time, I'd see about switching to rats, because several mice have a lot of bones and less meat than one rat would have.
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