Just some syntax to pass along for those who may wanna reformat credit cards (if you are brave enough to store them as plain text in the first place) before sending them to an application for displaying. This lets you pad out all the numbers except the last four (all you developers have done this millions of times I'm sure.)

SELECT LPAD(SUBSTRING(`cardnumbercolumn`, -4, 4),LENGTH(`cardnumbercolumn`),'*') FROM table