@There's nothing wrong with the antenna
Yes there is. The open circuit end of a monopole antenna is high impedance (zero current because you've run out of element, and thus highest voltage and *very* high impedance (Z)).
Because the end of the monopole is hi-Z, it is very sensitive to being loaded-down and/or detuned when touched. This hi-Z end of a monopole antenna is normally (instinctively) kept out of reach. The Iphone 4 has two external monopoles (differently tuned) with their hi-Z ends right next to each other where they can be touched, effectively bridging them.
It's a case of YMMV. If you have very dry hands and live in an area with strong signals, then it may never be a problem. But if you have hands that are damp with salty sweat and are in the fringe of coverage, then one false move and physics dictates that the signal will drop.
The wrap-around antenna is nice in some ways, but the design should be modified. One option would be to extend the monopole elements, bending them within the case (effectively end-loading). The hidden end would perhaps have to be effectively length-entended with dielectric. This would bring the hi-Z ends of the elements inside the case where they couldn't be touched. The part that remains outside would naturally present a lower impedance and would thus be less sensitive to being touched.
If anyone with any common-sense knowledge of antennas whatsoever had been in the room when the internal design of the Iphone 4 antenna system had been unveiled to management, they would have *immediately* pointed at the hi-Z ends and said, "You'd better insulate those ends..." I estimate the reaction time would have been about 1 or 2 seconds.