My Ibook G4 has fallen and the screen is now only usefull/visible at the top left corner. I heard that there is some sort of cable to hook up another computer monitor via USB or something. Does any one know anything about this and could you help me out?
You do not require any additional cables. The monitor should have a cable coming out of it that will connect to the back of the G4 laptop. Depending on which type of monitor it is, it may not work with that G4.
Refer to this webpage:
http://docs.info.apple.com/article.html?artnum=86283
Depending on what picture looks like the back of YOUR g4, you might either have a VGA port or a DVI port. LCD monitors commonly use the DVI port, while CRT monitors use the VGA port.
However, laptops do not usually start using monitors automatically. Usually they require a keystroke to activate sending data to the monitor. I'm unsure what this keystroke is on the powerbook.

You do not require any additional cables. The monitor should have a cable coming out of it that will connect to the back of the G4 laptop. Depending on which type of monitor it is, it may not work with that G4.
Refer to this webpage:
http://docs.info.apple.com/article.html?artnum=86283
Depending on what picture looks like the back of YOUR g4, you might either have a VGA port or a DVI port. LCD monitors commonly use the DVI port, while CRT monitors use the VGA port.
However, laptops do not usually start using monitors automatically. Usually they require a keystroke to activate sending data to the monitor. I'm unsure what this keystroke is on the powerbook.
References :