I think it depends on how strong your arm/wrist is and how good your technique is. I personally always go for 3U (even for super head heavy rackets), as I have a strong arm/wrist and I like smashing and breaking the opponents' defense with raw speed. For me, this is by far the most enjoyable aspect of badminton. In Physics principles (keeping in mind there are numerous other variables), Force (F) = mass (m) x acceleration (a) Therefore, a greater mass will produce a larger force for the same acceleration. The problem is that some people aren't able to apply the same acceleration to their racket swing with heavier rackets.
Quick answer for a quick question: Held everything else constant, 3U is more powerful than 4U, but 4U is quicker than 3U.
