The 4th diagnostic LED is tied to the display so if you have it disconnected it won’t lite up which would be to correct response. [guide|7443]
[quote]***LED 4*** - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.[/quote]
If your sure you got all of the display cabled connected properly and they are working, then its tell you the T-CON board within the display has a problem or the Inverter.