Difference between iPhone SDK 3.x and 4.x

Question

• what is the difference between
  iPhone SDK 3.x and iPhone 4.x ?

• what is the new features in iPhone ?

• How to feel a iPhone programmer the
  iPhone SDK 4.x feature. ?

Thanks & regards.

Answer 1

See the "Whats new in iOS 4" documentation from Apple:

http://developer.apple.com/library/ios/#releasenotes/General/WhatsNewIniPhoneOS/Articles/iPhoneOS4.html#//apple_ref/doc/uid/TP40009559-SW1