Hi chip0004,
Diagnosticl LED 4 in XPS 400 system indicates an error with BIOS checksum. You may try draining out the residual power from the system by following steps below and see if it helps :
- Power Off the computer.
- Disconnect all the cables and peripheral devices (printer,flash drives etc) from the computer
- Press and hold the power button for 4-5 seconds so as to drain the residual power from the computer.
- Connect all the cables to the computer again and try to power on the computer.
If the system powers on, you may try running diagnostics to verify the hardware functionality :
- Power on the system
- Press F12 button on keyboard when you see Dell logo
- Highlight Diagnostics option and press Enter
- Let the diagnostics run and reply with error messages if any and continue tests
Hope this helps.