When booting I do not get a display and the # 4 is lit

o F2 boot to BIOS.
o F12 boot device menu.
0 Fn+F1 sleep/hibernate per power management setting.
o Fn+F2 battery charger ON/OFF.
o Fn+F3 DCP Power Manager Battery Status.
o Fn+F4 numlock ON/OFF for embedded Fn+numeric keypad.
o Fn_F5 scroll lock ON/OFF.
o Fn+F7 DCP.
o Fn+F8 display device.
o Fn+D display OFF/ON (conserve battery without shutting down entire system).
o Fn+Z reset fan temperature table and sensor.
o Fn+Right-Arrow keyboard illumination OFF/ON Auto ALS (lit upon input in dim lighting)/ON (lit upon input). Use DCP to set input device, brightness, and auto OFF delay.
o Fn+Left-Arrow display Ambient Light Sensor OFF/ON (brightness varies with ambient lighting).
o Fn+Up/Dn-Arrows override display brightness until boot to BIOS setting and/or power mananagement setting.
o Alt+Fn+F11 screen shot.

• rjsymmes has the most accurate solution. Start with a battery replacement. Next course of action is to use a Bootable Media Source (Windows Disk or Recovery Disk) to see if the laptop responds and can have the Start Up repaired. If a Boot Media fails, then examine RAM, Hard Drive removal, then boot. Last resort Motherboard.

look here
DELL Dimension 9100 No Booting diagnostic light 1 green 2 3 4 green

Also bear this page and links in mnd

If the system boots, check the BIOS overheat settings.
If no boot, Most common problem, BIOS is corrupted.

Look at the power indicator:
If it is not lit, no power is being supplied to the computer. If the computer has a battery, it may be low and unable to power the computer. Connect the AC adapter to recharge the battey pack and make sure the AC adapter is properly plugged into the computer and to the AC power outlet.
If the power indicator is lit, check if a non-bootable (non-system) disk or USB thumbdrive is in the computer. If so, remove it and press Ctrl + Alt + Del to restart the system.

Pressing F2 by itself is only used to enter the system's BIOS on startup. If the power indicator is lit but nothing appears on the screen - The computer's power management system automatically blanks the screen to save power. Press any key to turn the display on.
If pressing any key does not turn the display back on, check the following....
1. The brightness level might be too low. Press Fn and the right cursor key to increase brightness level
2. Display device might be set to an external monitor. Press Fn and F5 to toggle to the internal display
3. Backlight might be turned off. Press Fn and F6 to turn on the backlight.
4. If the Sleep indicator is lit, the computer is in Sleep mode. Press & release power button to resume.

If it boots normally and there is no display on the monitor,it could be

1.Your Display card could have failed or the VGA cable from the display adapter is not connected properly to your monitor- In this case you will see a "no signal" message on your monitor.

2.If the monitor is powered on nothing appears,then your monitor most likely failed.

3.If the monitor's power on led light is not lit,then make sure your monitor is plugged in properly to the power source.

Try re-connecting/re-seating the video cable/connector again.

The power supply is most likely dead.

Open this link and make it full-screen. Watch the screen for 20 seconds or so to see if the line of lit (white) pixels remain.

If this line remains during this, and even when the the computer is first starting up, it is most likely a row of "lit" pixels caused by a faulty connection or circuit inside the display.

While you can sometimes rub the area of the screen near a single lit pixel, an entire line of them usually denotes a more complex problem that can only be fixed by having parts replaced by a technician

Note: Be sure all of your cable connections are securely fitted, and that the line is there no matter what is displayed on the display (especially at the boot screen before Windows loads).

its a sensitive problem, 1st remove your CD-Rom drive then start your laptop, if consist then open the skrews of hard disk portion and remove it also then open your laptop it will display the screen.
otherwise contact your hardware doctor.
shafky