unless you are burst shooting and running up your buffer, its really not that big of a deal what card you use.
I got a bunch of sandisk pro class 10 cards (30mb/s) and a couple of the UC-1 sandisk cards (45 mb/s)
in 8-16 gig varieties. probably way overkill for the 2-3 consecutive shots I might take at any given time.
the regular class 10 cards are so cheap right now i don't see much point in going any slower than that, but unless you are burst shooting, i doubt you would see any difference between the class 4 and the class 10 cards unless maybe you were using a D800 in 14 bit RAW pushing out 50 meg files.
my D7100 does around 25 MB raw files and i cant shoot fast enough that my regular class 10 cards cant keep up.