If you're good at 3U then definitely try 4U. Even if you're not too sure about it, you should at least try it, then you can drop it later if you need to. But, like everyone else has said, it's not
too hard (some topics are easier, and some are harder), but you have to have pretty good organisational skills to deal with all the homework and extra work that you have to (or should) put in. And if you want to do well, then be prepared to spend long long nights doing many many past papers and questions, because the people who do this always get the best marks. But, if you tend to be lazy, disorganised (i dont mean messy, but structure/time-wise), or, well, lazy, then perhaps 4U isn't the right way to go.
And it scales pretty well too.