4k is WAY more dense for each finger than 7k. if you look at a 4k 5 star youll see each finger is tapping way more than a 7k 5 star, doesnt have to be 5 star it can be just about any star rating
edit: not sure about <4 star songs because higher key counts tend to use alot more hands (3 notes at same time) than 4k so for lower star ratings 7k might have higher notes per seccond, but take that with a grain kilogram of salt because this is a guess