I'm also in S4 and my prelims are at the end of january. I have started studying now, and the best way to do it is to make a timetable that suits you. For example, say you wanted to do three subjects a day for one hour each that would be enough. However the days you know you dont have much on you can add maybe one or two more subejcts to that day to study for an hour each. It can be quite confusing and three hours a day may not seem like much if you are like me and you stress all the time. But remember, don't over do it. Then you may be burned out after your prelims. The exam is the most important thing.
My brother and sister are both studying medicine and i would advise you crash biology because it is very important to have. It's an easier science than physics and chemistry (if you have a good memory).
English is a hard one that I'm not amazing at as well, but to get the 'A' just practice tons of close reading and get maybe your teacher to mark it (thats what I do), with textual analysis it works differently for people, you can memorise your text or different quotes. The main thing is practice. Even wiht essays keep writing essays and giving them to your teacher.
Hope I helped (: Good luck!