uh, the weight of the 4g is heavier, so this translates to more force on the glass when dropped......
Lots of good theories on why it breaks more - all seem plausible. This my theory (i have a 3gs). The 3g construction with the plastic side and back has a little bit of flex, and can absorb some of the energy when dropped. The 4 has metal frame and glass back so it is much stiffer and apt to transfer energy to the glass and crack it. Also, the glas it self is a bit more exposed, so it is more likely that an impact will be directly on the glass.