Can I turn off the stove mid-way through making the beef stock? I didn't know it would take 4 to 6 hours!
Sure, turn off stove, chill your stock quickly, then refrigerate. To chill quickly, you can fill a sealable container with ice, float it in your stock, and stir. If you can also simultaneously put the pot in an ice bucket, chilling will be even more rapid.
You can stop anytime you want really, but the end result is going to be vastly inferior. Beef stocks can easily go 7-8 hours and you will really notice the difference. If you are looking for a faster method, use an instant pot or pressure cooker, which will give you an 8 hour tasting stock in closer to 2-3 hours.