Hi, don't worry about how long it is. I've written messages which take up 2 full posts
You don't have to do 4 unit in year 11. Obviously, you do preliminary in year 10 which involves mathematics and mathematics extension.
In year 11 you do HSC level. Obviously doing HSC extension 1 and having done Extension prelim is a prerequisite to doing it in year 11.
Obviously if you did 3 unit HSC in year 11 then you could do 4 unit in year 12. Generally you would do 3 unit once again, so that you're refreshed.
Also, if you accelerate in a subject, then it counts towards your HSC. You can do your HSC in a total of 5 years. And you can redo it, but the most recent mark will always be taken for ATAR calculation.
Hope that helps.