Pork belly in a crock pot?

With cooking any meat, you are trying to balance the tenderness and juiciness. With braised pork belly you are trying to achieve a tender meat (cooking for a long duration to break down collagen) while retaining moisture (not cooking the meat at too high a temperature and for too long a time). Just getting the belly to internal temperature is not enough, you have to get it to maintain the temperature for some duration so enough of the collagen breaks down into elements that contribute to tenderness and moistness (including gelatin).

Most people will braise or cook sous-vide the pork belly at a temperature between 80 degrees to 90 degrees celsius (which is pretty much a low simmer) from between 3-9 hours. That gives it enough time for the meat to tenderize while cooking it at a low enough temperature to not dry the meat out (as proteins denature and bunch together from cooking water will slowly be squeezed out of the meat. If the temperature is too high for too long you will end up with dry meat fibers).

Putting the belly in the crock pot for 4-5 hours will be perfectly fine, you'll get a great result. Don't just cook it until the internal temperature reaches 80... which shouldn't take more than 30 minutes or you'll get a gray piece of tough meat.