Aren't you ignoring the position requirement? They can't be just random 4 bits out of 32, they must be in a sequence.
no, the other 28 bits are inconsequential and don't even have to be considered.
also, consider this: the chance that the upper 4 bits are all 0 is the same as for example bits 3, 9, 13, and 30 all being 0.
(again, assuming the RNG gives uniformly distributed numbers)