Is cooking a pot roast 3 hrs and 45 mins on low and then an hr and 15 mins on high enough time?

Pot roasts are tricky in slow cookers. I don't go by temp, but when they are tender. They can be stubborn huggers. It should be good with your plan, but it also depends on your particular slow cooker. I have 3 and they cook at 3 different temperatures.

Depending on the cut, but assuming it's a typical one used in a pot roast/braise (e.g., brisket, chuck, etc.) I'd be less concerned about internal temp after 3 hrs (never hurts to check though) and more focused on how tender it is. If it's good and tender, it's ready to go. Generally speaking, a blast of high heat runs counter to 'low and slow' - the method that makes a pot roast sing.