The problem in answering this question is that nobody has done both 4 A levels and 4 HLs, so I guess it's easy to be biased. But in order to do a STEM career, you would need HL math, which is insanely difficult, plus other science HLs, which are also difficult. IB won't let you concentrate on these - instead, you'll just get bombarded with english literature, foreign language acquisition, EE, IAs, CAS, TOK, etc. In my opinion, A levels would just let you concentrate on these difficult subjects, giving you an adequate amount of time to do them. I got 43 points and I can safely say doing 4 HLs would have absolutely killed me. The IB's difficulty is really dependent on what HLs you take, but given that you want to take the hardest ones, 4 would be suicide, in my opinion.