Does Cambridge care if you do 3 a levels compared to 4? I have heard a lot of their students do 4 but still get really really good grades in them
Do 4 if you are doing Further Maths, else do 3, is the general recommendation.
Doing 4 does not give you an advantage for getting in, you often end up with a 4 grade conditional offer, so you can't then drop one of them, you have less time to do super curriculars, and of course you are spread more thinly so risk dropping grades in the critical subjects.