hi! first off, congrats on your summer results, they're great!
i am currently in y13 and take chemistry, physics, maths, and further maths
depending on your exam boards, each one can be different - one of my close friends goes to a different school and started with the same subjects, wanting to focus on maths, but her exam board was awful for further maths and she ended up dropping it.
if you're passionate about maths, i don't know if this would be a problem for you as you would just go with the flow and take the content as it comes up - i am quite like this but there is definitely a jump in difficulty as you progress through the course. y12 for me was fine, but as i've come back for y13 the content has gotten significantly harder but i do enjoy it so i am sticking with it.
i cannot comment on computer science as i genuinely have not had a lesson since year 9, but chemistry is a bit of a jump from gcse straight away and the content gets progressively harder. it is my least favourite a level at the moment but was my favourite gcse - so bear in mind that your opinion on your subjects WILL change, this could be because the course gets harder or because you just lose the passion for it.
i still think that the chemistry a level is very interesting, but there is a LOT of organic chemistry, which does mean that there's a lot of memorisation and it can get repetitive and confusing as they are fairly similar. this would make up almost a full a level paper so also think about that when looking at this a level. it is also extremely content heavy - i have 4 lever arches for chemistry content alone and we are not done with the spec yet, while i am not halfway through my 2nd lever arch for physics at point of writing.
physics is a great choice at a level but i am definitely biased as i am taking that forward at degree level. i would say that it is fairly similar to gcse in terms of the exam style and how the content is taught, but the content does get harder as you would expect with an a level. i personally love that it is quite maths based, and from what i can understand about you, you would also enjoy that aspect.
my best friend takes biology, chemistry, and maths at a level and she loves biology but it is very content heavy, a little more than chemistry. a common a level combination is biology/chemistry/maths/further maths, but that would be more likely if you are going to take a biological sciences route, and you're interested in that kind of science.
i completely understand the problem of a noisy family - mine is the same - but i find that going to the library on a saturday basically sorts out any of the backlog of work.
also a note - taking 4 a levels is a lot of hard work, and i have no doubt that you wouldn't be able to do it - i dont even know you! - but if you aren't too sure on a 4th, there's no shame in dropping it as it will mean more time to dedicate to your other 3.
hope this helps even a little bit and sorry its so long!! good luck with your gcses