A 4 pound snake cannot push over a heavy space heater, but they sure can crawl on it, or even into it in some instances.
We already know they aren't the smartest to move off of objects that are burning them. Space heaters get HOT while they run. Very hot.
I'd be even more afraid of finding the rat AND snake gone, come morning. Snakes can disappear into the smallest of spaces.
Just not a good idea to let a snake loose, after a loose rat. IMHOSet a trap. Doesn't have to be a cruel trap, if you want to catch the rat alive.