bad bad misconception. just because you do 4u doesnt mean u can do the 3u extension questions. in fact some of those questions r so ridiculously insane our teachers cant get it out. especially some of the perm/comb ones. so dont think you cant do extension questions doesnt mean u cant do 4u because those extension questions are 4u level + logic or think outside the box ideas
Depends on what mark ur aiming for for 4U-but i would suggest that if u want a decent mark for 4U u should be able to do most of them(emphasis on most and not all)