4th led light will not turn-on

I replaced my 1TB hard drive,and after replacing everything back as instructed my monitor remain black, I was advise to check the led lights associate with the operation of the computer system, there are 4 such led lights, the first three light-up when the computer is first turned-on, but the fourth led remain off, My question is , should the 4th led light-up with the remaining led lights when the computer first turn-on.?

what could cause this problem?.