This letter is about 200x more important than any of the other letters you have. It doesn't so much "eat up a slot" as it "allows you the possibility of moving on with a career in medicine." Seriously, without this letter (assuming it says good things about you), nobody will care how many other letters you have, or how good they are.
My suggestion is to pick the 3 you think are the strongest and go with that. 4 LORs is more than enough.