I made some research on this and found http://arxiv.org/pdf/physics/0210022 which says basically that drop rates among 720 different carriers have a lognormal distribution with mean of about 2.4%.
iPhone 4 might have better RF and baseband chain to compensate, but it's clearly not enough when the phone seems to lose the signal altogether.
Edit: Jobs said clearly that the 4 drops ~1 calls more per 100 than the 3GS. 1 per 100 -> 2 per 100.