Q: I need to know how to unlock my iPhone screen. i have an apple 4G

If you've forgotten you passcode : If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

If it's on the activation screen then type in the id and password of the account that was being used in Settings > iCloud on it

If the phone is locked to a carrier then contact that carrier and see if they will unlock it

If you mean something else ... ?