Should I take 4 A-Levels?

I currently study biology, chemistry, religious studies and psychology at school, as well as an extra language at home. I am a native in the language and have no struggles with it whatsoever so will take the exam during year12 (predicted A*). I'm predicted As in all my other subjects and currently working at an A* in psychology and have learnt most of the course already in my own time. I cannot drop biology or chemistry as my parents would like me to take medicine, and those are non-negotiable subjects. I enjoy R.S. with a passion and dropping it would mean I have no creative subjects (or even subjects that I enjoy) left, so I'm hesitant to drop it. I am also hesitant to drop psychology as I have a natural affinity for it and it feels like an easy A*, which is appealing. Is it worth taking all 4 (for the sake of my own mental stability to have something I enjoy) or is it genuinely unbearable ? I would be willing to dedicate most of my free time to studying, as I don't have a job or any extracurriculars. Is the transition from yr12 to yr13 bad enough that I would end up regretting this decision? Does it give me any kind of advantage at all ?

Unless you are someone taking Maths and FM and heading for a Maths orientated STEM degree, the simple answer is NO.
For Medicine and the majority of related degrees, no University requires or prefers more than 3 A levels - they do not give you any extra brownie-points or think that you must be 'extra clever' so its totally pointless doing an extra qualification that is not going to make any difference to your UCAS application. If anything, the extra work, effort and stress is going to detract from all your grades - remember AAA will always look better than ABBB, and that could cost you a Uni place.
If your parents are bullying you into Medicine, or into taking 4 A levels, talk to your Head of Sixth or similar and ask for their help/support. Remember there are plenty of other courses and careers - Psychology, Pharmacy, Biomed or Chemistry research, Law, International Relations etc - that are entirely possible with your A levels.

I currently study biology, chemistry, religious studies and psychology at school, as well as an extra language at home. I am a native in the language and have no struggles with it whatsoever so will take the exam during year12 (predicted A*). I'm predicted As in all my other subjects and currently working at an A* in psychology and have learnt most of the course already in my own time. I cannot drop biology or chemistry as my parents would like me to take medicine, and those are non-negotiable subjects. I enjoy R.S. with a passion and dropping it would mean I have no creative subjects (or even subjects that I enjoy) left, so I'm hesitant to drop it. I am also hesitant to drop psychology as I have a natural affinity for it and it feels like an easy A*, which is appealing. Is it worth taking all 4 (for the sake of my own mental stability to have something I enjoy) or is it genuinely unbearable ? I would be willing to dedicate most of my free time to studying, as I don't have a job or any extracurriculars. Is the transition from yr12 to yr13 bad enough that I would end up regretting this decision? Does it give me any kind of advantage at all ?

I'm currently in Year 13 and went through a pretty similar thought process to you (though I'm going to do Psychology and Language Sciences at uni rather than med). I take Psychology, Biology, French, Classical Civilisation (AS), and I did Japanese in Yr 12 (achieved A*). If you have a passion for RS and feel Psychology will go smoothly, I'd say it's probably better to keep all of your subjects. Though make sure that you actually want to go down the med route bc as you can probably imagine, the work doesn't stop at A Level, and it will likely be mentally draining if you have to do the same stuff as ppl who have always aspired to be medics at uni without external pressure. I'm predicted 3A* and an A (AS) and used to go out a fair bit in yr12, so I can assure you that it is possible to stay alive while doing more than 4 subjects, but even though the early A level is in your native language, there will likely be some work needed since mark schemes can take a non-native stance on things, so keep that in mind. + The workload can sometimes be overwhelming in yr12 (bc of the extra A level), and I had my actual exams for Japanese in-between the mock exams for the other subjects that determine predicted grades - which go to uni, which was VERY tiring, stressful, and can potentially cause mental breakdowns (make sure to have plenty of naps in-between)...Yr 13 does get busier with uni applications, more content etc, but there is a little less pressure since you will already have an achieved grade (keep at it to make sure you secure that A*! ). It'd likely depend on which Unis you're interested in as well since although most say they only look at your top 3, places like UCL will sometimes take the extra one into account (if it's achieved) and lower the requirements accordingly, which is defo a benefit. Oxbridge don't really care about extra A levels, but there is a tendency for successful applicants for med to have Maths and 4+ A levels bc it's so competitive, so have a look at requirements at different unis to check you can apply to them. Quite a few of my friends have applied for med, and a lot of them are doing Psychology A level - and have gotten offers after their interviews, so it might be worth keeping it as long as you incorporate it well into your personal statement when you come to write it since it'll show that you can look at the discipline from different perspectives (likely the same idea for RS). The other thing is that your subjects are/will be fairly linked since there are some crossovers between Biology and Chemistry/Psychology (esp. biopsych), so there will be a tiny bit less to do than usual. Hope this helps.