The 4 and 4S had separate structure parts and they never bent.
gee, i wonder what could have been different about the 4-series? could it have been...they were 4.5" long and .37" thick, while the 6+ is 6.2" long and .28" thick? now, what do we know about length and levers.... oh yeah! the longer they are, the more force they generate when positioned against a fulcrum...
http://en.wikipedia.org/wiki/Lever