the only time i think 4 a-levels can be helpful is when you're applying for courses where there are a lot of subjects that feed directly into it at very competitive universities. i've got an offer to study chem at oxford and aside from the required maths and chem, bio/physics/fm all provide really good preparation for the course and so having studied these subjects may make you slightly more desirable. that said, some colleges/schools can't even offer students the chance to do 4 so it would be unfair to give any significant advantage to these candidates and so many people who will be on my course have only done 3 a-levels. having done 4 a-levels the last 2 years i can honestly say that regardless of my grades i'm glad i did it because i have gained so much knowledge which will stand me in really good stead for university and it's massively improved my time management. however, i don't generally recommend it because it just doesn't give you any real advantage, and none at all in the majority of situations. to do it i think you have to actually want to study the 4, and not just be doing it to make yourself seem like a 'better candidate'. remember that by doing a 4th a-level, you aren't just increasing your workload, you're also decreasing the amount of time you have to complete said increased workload (time spent attending lessons in the 4th subject takes away from how much time you have to study outside of lessons). if you're organised and meticulous from the start you can definitely do it if you have the aptitude but i'd say 99 times out of 100 it just isn't worth it!