using string format for a credit card number

String.Format("{0:0000 0000 0000 0000}", number)

EDIT

I paste my comment here to made it readable:

ccNumber is an Int or a string? if it's an int it should work. if it's a string you need to do a

String.Format("{0:0000 0000 0000 0000}", (Int64.Parse("1234567812345678")))