Particularly, read the second page about iPhone 4 antenna. In short it can be concluded, that Apple somehow calibrate the bars too optimistically, probably because iPhone 4 is able to work at places where signal is very low. This problem is probably solvable by software update.
Second conclusion is that holding the iPhone 4 in certain way is in fact causes more problem compared to other smartphones. This is unfortunately won’t be solved just by upgrading the software.