Touchscreen works, but no display.

I'm currently trying to fix an iPhone 4 that had a cracked screen. At first I tried to replace the iPhone 4 screen with a spare iPhone 4S screen, which actually showed the logo upon boot, but would flicker during boot and just quickly flash when trying to unlock (ie. pressing unlock would turn the screen on, but only for less than a second, giving me a chance to see the lock screen wallpaper).

Then I bought a new screen made specifically for the iPhone 4, but it was much worse, the logo does not even show up during boot, and when I press the unlock, it lights up for less than a millisecond.

I've searched around, and saw someone mentioning that the screen I have bought required me to update my phone. Unfortunately, my wife put a passcode on it which she forgot (we cant even see the screen in the first place) and the original laptop I synced the iphone to is long gone.

I can confirm the touchscreen and some of its components work since (for BOTH screens):

*I can attempt to find the "slide to unlock" and I can hear the keypad click.

*I can take screenshots using the home and lock button

*I can also switch back and forth the vibrate button.