If you do 4u, there's a pretty high probability that no matter the mark you get, considering the double weight of 3u now and the fact that 4u scales and aligns lake crazy, you'll very probably come better off than if you'd not done 4u, but simply 2u and 3u instead. And that's really a worst case scenario.
4u is very valuable course if you go on to do any form of mathematics at university.
This all assumes of course that you have at least an average amount of ability at 3u in year 11.