Just out of interest: Do you think it would be better to focus on chem or bio for top 4?
Try extremely hard in both ASAP in preparation for the midyears. After them, get a statement of marks for both and see which one you did better in. Dump the one which you did worse in, or both if you don't think either are top 4 material (only if you're confident in your other 3 though of course).
EDIT: This is the trick that I did at least. Spent the entire first half of the year on Chem, got an awesome midyear score (lost 2 marks or so), and then neglected the subject for the rest of the year and crammed about 5 practice papers 2 days before the end year. Did fairly 'average' (8-10 marks lost), but still did well due to my midyear score and my SAC marks. Consequently, I had more time to spend end year on my other subjects. Despite what people say about consistency throughout the year, the home stretch of the few weeks just before the exams really is the most vital time by far in terms of retaining knowledge and whatnot, and I really couldn't find the time to do all 5 subjects there so ignoring Chem really helped.