Macro for removing all numbers except last 4 digits.

I have a problems and have looked everywhere to find a solution. Here is my situation:

I have a spreadsheet the list social security numbers. Some cells are blank. I have a macro that will convert the number, but when it hits a cell with nothing in it, it past in the macro as a answer.

How can I tell it to skip the cells with nothing in them?

Thanks.

I was looking right at it and missed it.

This does great. Thank you.