SELECT RIGHT(MyColumn, LEN(MyColumn) - 4) AS MyTrimmedColumn

Edit: To explain, RIGHT takes 2 arguments - the string (or column) to operate on, and the number of characters to return (starting at the "right" side of the string). LEN returns the length of the column data, and we subtract four so that our RIGHT function leaves the leftmost 4 characters "behind".

Hope this makes sense.

Edit again - I just read Andrew's response, and he may very well have interperpereted correctly, and I might be mistaken. If this is the case (and you want to UPDATE the table rather than just return doctored results), you can do this:

UPDATE MyTable
SET MyColumn = RIGHT(MyColumn, LEN(MyColumn) - 4)

He's on the right track, but his solution will keep the 4 characters at the start of the string, rather than discarding said 4 characters.