No need to use a macro. Supposing your first string is in A1.
=RIGHT(A1, 4)
Drag this down and you will get your four last characters.
Edit: To be sure, if you ever have sequences like 'ABC DEF' and want the last four LETTERS and not CHARACTERS you might want to use trimspaces()
=RIGHT(TRIMSPACES(A1), 4)
Edit: As per brettdj's suggestion, you may want to check that your string is actually 4-character long or more:
=IF(TRIMSPACES(A1)>=4, RIGHT(TRIMSPACES(A1), 4), TRIMSPACES(A1))