For instance: When I have info in CELL B4 I need an auto generated number in CELL B5 starting with 2000
CELL B4 CELL B5
PFP 2000
PL 2001
STD 2002

I think you meant when you type in B4, you want a number to appear next to it in C4, yes?

Put the first number 2000 in C4 manually.
Then in C5 put this formula and copy down as far as you wish:

=IF(B5="", "", C4 +1)

I am trying to auto number cell d4 when info is put into c4

I am trying to auto number cell D4 when info is put into cell C4

Adjust the formula given to the references in your columns. That formula watched column B and was placed in column C, so give it a try. What would the formula change to if the formula was in D4 instead?