All 4 children can stay together, and for in-room sitting, they give a per child discount once you are up to 3 or 4.
Some of the Kids clubs are open to anyone staying at a disney resort. So, if you were planning an adult nite out and eating at Artist Point at WL, the kids could stay at the Cubs Den there. You would need to call Disney to find out exactly which ones.
Down side to the club's are that they close at 12:00, so if you were planning a late night at PI, that would not work. Plus, if you are taking Dis Transportation, getting from resort to resort can be challenging.
I, personally, have only used the kids clubs. However, I've been researching in-room for this trip. The in-room sitters are bonded, carefully screened and all non-smokers. They will even take the kids to the arcade or the playground, etc., if you request it. Plus, the old adage, safety in numbers. With all 4 kids there, and the oldest being 9, I don't think you have to worry about their safety with a sitter you don't know. I read a handful of positive reviews about sitters - nothing negative - that people had posted. If you do a search for babysitting, you should be able to find some older discussions on this.