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?.

I'm sure someone will point out similar questions that have been previously answered.

Briefly, from a food-safety perspective, I would NOT recommend it.

Bacteria grow in food while in the "danger zone" of roughly 40F to 130F. You're proposing to take three trips through that temperature range instead of one. That increases risk and is easier to mess something up. (This includes growing bacteria or persistent toxins that may not be destroyed even if the food reaches a high final temperature.)

I could describe the detailed food safety regulations you'd need to follow to do this, which would include time and temperature monitoring during heating, cooling, and then reheating. But if you're new to cooking, I wouldn't suggest this. Commercial operations are equipped with things like special chillers to cool food rapidly. As a home cook, the FDA and USDA would instead recommend that you cut up your roast and cool it in pieces in shallow containers to chill rapidly.

And that's the only way I'd try this at home unless you really know what you're doing. Be sure the center of the roast has cooked to above 130F (probably at least 140-145F for safety), then cut into small pieces, cool as fast as you can, then reheat quickly so you get to 165F within a couple hours (which may not be guaranteed in a slow cooker).

HOWEVER, since it sounds like you want your roast to be cooked and ready around noon, I'd recommend just cooking it on low overnight. If you leave meat in a slow cooker for 12 hours on low instead of 8 hours or whatever, it will become more tender, but that's often what one expects in a slow cooker.