Are you talking about next year as in January? Or as in after your HSC, as in year 2008? If the former then it should be counted as any other course should, but it would be advised to do it within the school as you can ask for help anytime
But, if you mean studying it in 2008, then:
If you have at least 10 units now not including the 4 unit maths you want to do, then the 4u maths course next year, I don't think will be counted as you already have 10 units which is illegible for a uai. Why not try doing the 4u maths now so even if you stuff up you can still drop down if you're feeling uncomfortable.
If you want to do it next year, then it would be probably done as a leisure course more than anything. In university you will learn the maths you learn in high school including some 4u maths.