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While I didn't do 3U last year, I did do 2U last year and am currently doing 4U this year as well as 3U. While if you do 4U, as well as 3U and 2U, your 4U will be worth 2 units, and your 3U will also be worth 2 (essentially discounting your band 6 in 2U; it is true as far as I know), the payoff is very worth it.
There's a couple of reasons that I've found made 4U worth doing despite accelerating 2U:
- You get more units of maths counting to your ATAR (four units as opposed to two/three) - and clearly it's a very strong subject for you, so that's definitely a good thing
- While 2U won't count, and it might seem like a waste in the meantime, it does put you ahead of much of the 4U cohort in the state that hasn't managed to complete the 2U course prior to doing 4U - 2U is a good building block towards doing 4U (the argument here is that while 4U is a step up, it's less of a step up than it would otherwise be for many people - essentially you will have time to focus on your other subjects)
- Also, while 4U is a step up, once you've gotten past all the content, armed with your new knowledge, I've found that 2U this time last year and 4U today varied in difficulty very little. It's definitely worth doing, and you should get similarly strong marks
- If you back out now, you probably won't have the option to try it again later in the year - try and it and see if you personally like it; if not, drop it
- I've saved this one for last because while it's important, it's definitely not fundamental and not something you should focus on. Scaling in 4U is better than 2U, and judging by your talent, a few extra marks dropped in 4U won't make a significant difference, and perhaps could even be better. And who's to say that you'll even drop those marks, given the work you can put in over the next 12 months? You have plenty of time to develop and succeed in 4U. It's definitely not as straightforward as 'my 4U mark was worse than 2U, 2U won't count' - remember that your ATAR is taken based on your 2 best units of English, then your next best 8 units.
Ultimately while these all point towards you continuing and then making the decision later on, it really is ultimately up to your personal preference. It sounds like a pretty crappy answer but the easiest way to succeed in the HSC is tailoring your study towards your strengths, and from what it sounds like, it lies in maths

Sorry for the long read, but I hope you found this handy!