I forgot my Iphone 4 passcode - Apple iPhone 4 Black Smartphone

SOURCE: I forgot my passcode to unlock my iphone

It depends on the severity and the settings you have given the iPhone and whether you have the computer that you synced your iPhone with... Here's the full explanation. I would recommend reading the second paragraph first, but the solution if you really forgot your password is in the first paragraph. 
==If you have the computer you synced your iPhone with==
If you cannot remember the passcode, you will need to restore your device on the computer that was used to originally sync the iPhone or iPod touch so that you can reset your passcode and resync the data from the device (or restore from a backup). If you restore on a different computer that was never synced to the iPhone or iPod touch, you will be able to unlock the device for use and remove the passcode but your data will not be present. Refer to "Updating and restoring iPhone and iPod touch software."

"If you enter the wrong passcode too many times, you'll see a message that says "iPhone is disabled" and to try again later. If you repeatedly enter the wrong passcode, iPhone or iPod touch will be disabled for longer intervals, before you can try again. After too many unsuccessful attempts, you won't be able to try again until you connect to the computer you normally sync with. Note that starting with iPhone 2.1 software, there is a setting to cause the device to erase itself after 10 consecutive incorrect password attempts. This setting is off by default. It  can be turned on by tapping Settings > General > Passcode Lock after you enter a correct passcode." 
The full article can be found here: http://support.apple.com/kb/HT1212
==If you don't have the computer you synced your iPhone with==
If you don't have the original computer the iPhone synced with, the only solution is to force your iPod Touch into recovery mode and then restore it. You will lost all the data on the iPod Touch, but after this method is done, everything will work. You can find the steps to solving this problem here: 
http://support.apple.com/kb/HT1808?viewlocale=en_US

SOURCE: i forgot my apple iphone

Try the Restore procedure from iTunes If you have problems. But first try the recovery procedure.
1.
Disconnect your iPhone cable but leave connected the other USB end to the computer.
2.
Turn off your iPhone holding the Sleep/Wake button. The red slider bar appears. Slide the bar and wait until the iPhone turns off. Or you could hold the Sleep/Wake and Home buttons at the same time until the iPhone turns off.
3.
While pressing and holding the Home button connect the USB cable to your iPhone. It should power on. Hold the Home button until you see the "Connect to iTunes" message. Open iTunes if necessary. You should see the "iTunes detected an iPhone in recovery mode..." alert screen.


Choose " Restore"