If the 4th subject is related to the area you're looking to study, then they probably would care. If you're applying for Maths and you get an E in AS Physics, it definitely wouldn't rule out an offer but it would raise eyebrows. Other than that, obviously it's better if you get a good grade, but I don't think they'd care too much.
The E at AS would fulful UCL's entry requirements of 3 A2s + 1 AS pass. Obviously, the above goes for UCL too.