Can I slow cook a 3.25 beef roast for 4 hours on high, refridgerate in the crock pot overnight and then cook on low again in the morning for 4 hours?
That would almost certainly violate a specific food safety rule. In normal use, a slow cooker barely stays within the safety parameters of the "time in the danger zone" rule, that says that food should not stay between 40F (4.5C) and 140F (60C) any longer than two hours cumulative. That means that preparation time, heating time, and cooling time all add up. That refers to the actual temperature of the food, not the surroundings.  In some cases you can go to four hours, but only if the food is eaten immediately, so that really doesn't apply here.

Whether that is overly conservative for you, only you can decide. Certainly many cooks would do exactly what you propose, without worrying about it all. It would violate safety rules for commercial kitchens in the US.

See the information here: How long can cooked food be safely stored at room/warm temperature?.