What happens is schools usually push kids to do 4 so that incase they don't like a subject they can drop it. I dropped bio after year 12. I would say do 4 just to be safe.
Universities see all 4 of your AS grades because you MUST enter them into UCAS even if you drop one. But unis only require 3 A Levels for entry. Your fourth grade isn't necessary as long as you get the 3 required grades.