No it does affect it. They divide the 3U cohort into people that do 3U and 4U and people that do 2U and 3U. If you get a good 3U mark, it will help your 4U mark. If you get a shit 3U mark, your 4U mark WILL BE UNAFFECTED.
Similarly for 2U/3U students, if you do well in 2U it will assist your 3U...