UCL Entry requirementsWatch
If you meet the entry requirement with the three A's then there is no issue in having an additional B. But if the B is in a subject they specifically require then you wouldn't meet their entry requirements with AAAB, so whether they could consider you would probably depend on the particular department and course.
Thanks for the reply…for entry requirements they mentioned Mathematics required, plus either Physics or Further Mathematics preferred. I have got B in Physics but Maths ,FM and Economics I have got A AA.