I did not read what other people suggested, but definitely take 3. Having 4 is useless in 99% of cases.
The only exception I can imagine is Economics at LSE, where FM is "recommended" (required in reality) as 4th (3rd could also be accepted) A-level. Other than that - no need.
I understand your willingness to take more workload to prove that you can do it, but it doesn't worth it. If you find yourself in a place where 3 A-levels ain't enough for your academic curiosity, great! You now have time to go after your passion, develop a new hobby or just spend time upgrading your social skills (I recommend the latest very very much). Doing 4 A-levels will not put you into any sort of advantage, but will cause you, perhaps, a lot more stress, workload and less time to spend on yourself.
I started of with 3 subjects at AS, and took Further Maths as 4th in January, catching up all of it by myself. Ended up having 95%+ in all papers across all 4 subjects. I didn't struggle to do it much, it came rather natural. I still dropped one of the subjects and ended up having 3 A*. I would have definitely gotten 4A* if I decided to keep my 4th subject, and I can't tell you how happy I am I dropped it. I had so much more time for myself, for friends and hobbies. I had greatest year of my life.
All in all, f*ck 4 A-levels, have some fun.