4 A levels or 3 A levels + EPQ

I think 3 A levels and an EPQ is better. You'll be able to demonstrate different skills through doing an EPQ and it can be a great way to show your passion for medicine. If you do 3 A Levels and get A*A*A* that would be better than getting a 4 A's. You don't get any extra recognition for doing 4 A levels, so it would be better to stick to three and do well in them.
Hope that helps!

Honestly I can't speak for Cambridge Uni because I haven't looked into it myself, but I'm sure that having a high score on the UCAT, doing lots of work experience/volunteering and a variety of extracurricular activities will be enough for you to stand out along with good grades.
What you could do is take all 4 at the beginning of the year and drop comp science if you find its too much. I'm taking biology, chemistry, and psychology and I think I'd go crazy if I was taking another subject lolll
Maybe it's worth speaking to a careers advisor head of sixth form at your school? They might have had students ask the same question in the past?
Hope it all goes well!

Thanks I was along the same lines as well tbh but I saw a video of this Cambridge med guy saying 4 a levels demonstrates 'spare capacity' or something so idk if that matters

I agree with prior advice. Take 4 and drop if it becomes too much. Ask in your prospective sixth form if it is possible to pick up an EPQ somewhere midway through y12. This gives you flexibility, since it means you have about half the year to decide if you want to drop, and you don't have to sacrifice the EPQ for this either in the case that you do decide to drop to 3.
Also consider very carefully what exactly your prospective degree course is looking for. Don't take any subject you are not moderately confident you will get an A* in. Additionally if you decide to take computer science, consider doing some activities that can link in and justify this choice with respect to medicine (e.g. you could try doing some supercurriculars related to computational biology or something as an example)