Just a thought for you to look into. Maybe they have an infection. if three are doing it and the 4th who stays away fron the others doesn't they may be passing an infection around. take the worst one to the vet for testing. I had this problem with house bunnies once and it was just that. It may take time for them also to get use to the box being cleaned more often ,so don't give up also when you catch them doing it pick them up and say no and put them in the box. Training a new behavior takes time and patince, breaking an old one is even harder.
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