These are diagnostic LEDs and when pc do not post they give a clue what might be wrong.  Normally these flash green then go off as system finishes post/boot.  If your pc posts well, no need to worry if one of the led stays on.
Hello! So i recently brought a Dell Optiplex 390 and i noticed when i powered on the system theres these lights that say 1,2,3 and 4 what do they mean? And what would i do if a one of the lights stayed on my computer?   Kindness, Jamie C  
Dell Optiplex 390 1 2 3 4 Lights?