I would personally advise against dropping biology - it’s going to give you quite a steep learning curve once you start your degree and is also likely to result in an offer with a higher grade tariff (check the unis’ websites for this). Not to mention for Leeds, I’m pretty sure you need to do the BMAT and continuing on with biology for A2 will definitely help with that. If I were you I would probably drop either maths or Spanish, whichever you enjoy least.
That said, I come from Northern Ireland where we do an AS in all our A-Level subjects in lower sixth and then sit A2 exams in upper sixth to convert them to the full A-Level, which gives us a lot more flexibility than some of the English schools where you sit everything in upper sixth. So I apologise if I’ve given you this advice only for it to be pointless.
If your only choice is between dropping biology and doing 4 A-Levels and you’re certain that the medical schools you want to apply to will accept you with AS biology then I would say that it’s just key to come up with an explanation for dropping bio that doesn’t sound critical of your teachers/school (even if the teaching quality is not what you hoped for) and trot that our at interview if they ask.
Best of luck