I did 5 in year 12 and dropped down to 4 in year 13. It was manageable, but I'd say probably not really necessary. I only took 5 as I really wasn't sure what I wanted to do in terms of university subject and so wanted to keep my options open. If you have more of a clear path, sticking to 4 is sufficient (then perhaps stay with 4 in year 13). If you're really uncertain about which field of study you want to go into, you can always start with 5 then drop to 4 halfway through the year if it's too much.