Cambridge GCSEs and A-Levels

For my GCSEs I recieved 999888776 and am taking Biology, Chemistry, Maths and Politics A-Levels but I am considering dropping Politics for either an EPQ or just doing three.

Are my GCSEs good enough for doing Medicine at Cambridge and should I stick with 4 A-Levels, replace Politics with an EPQ or just do 3?

Cambridge tend to not put as much emphasis on GCSEs and your GCSEs are very good anyway, so I don't think that's likely to be an issue in of itself based on the current format - might change in future with the BMAT going away though. The ATs who have posted in an official capacity for Cambridge in the past have all indicated they haven't previously rejected an applicant (for any course) solely on the basis of GCSEs.

In terms of your A-levels 3 subjects is all that is needed. Doing A-level Politics isn't going to give you any "bonus points" so you're probably better off dropping it to focus on getting the best results in 3 subjects that you can, rather than spreading yourself too thinly over 4 subjects. This will also give you more time to prepare for the BMAT (or depending on the year in which you are planning to apply, if it's no longer running, whatever replacement they may use). Note those only taking 2 STEM subjects and not 3 tend to be much less successful than those doing 3 STEM subjects when applying to Cambridge medicine, so I would recommend keeping maths if you want to apply to Cambridge (other medical schools don't care what your third subject is though).

Whether to do an EPQ is going to be based on whether you have a relevant project that fits the format of the EPQ in mind or not. No point in doing it if the only reason you're doing it is for personal statement fodder, as Cambridge don't make offers based on EPQs, and there are other ways to develop your interest and awareness of your subject besides that. So it's just something to do if you've got something in mind.