New iPhone has new quite ingenious antenna design - parts of the external steel frame are used as antenna, so there is a good change it will work better than the old iPhone.
They were both on wifi. The problem on the 4 was likely caused by it supporting N, which is of course much better than 3GS's G but is more sensitive to the edge case of ~500 people in the same room as you running wireless base stations without bothering to set up channels properly.
I don't think a change to the antenna would affect how it behaves in the same room as its base station, anyway.
