Chin, thank for your answer. I surely know iphone vs. its OS version, lol.
But, I don't get the point why I have to upgrade my phone to iOS5 while iOS4 still works very well. I know a new OS should have more features than the old one, but is always much heavier, more memory needed so often slower.
I hope OPMG can release its PG for iOS4 because not all people want to upgrade their well-working phones to a new OS; except it's required by Apple. IMO!