The only way you will know is if you try it. I've only ever used 3U. Haven't had the opportunity to try a 4U. Kinda not wanting to spend the money just to try a 4U. I suggest you try to borrow a racket of each type (preferably the same model of racket) and see which suits you more. Some say 3U gets you more power while 4U is quicker. So each will have some benefit.
Aside from the fact that one is 4U and the other 3U, there are no significant differences. The VT7 is supposed to have slightly better quality graphite in the frame, but that's all. Paul Stewart went so far as to say that they are so similar Yonex really didn't need to bother making both, and "there are such minor differences between VT5 and VT7 that I’d be hard pressed to tell them apart if the string tension was identical". Source: http://badminton-coach.co.uk/category/badminton-equipment/badminton-racket-written-reviews/
