If you use RIGHT([oldSSN],4) then the rest shouldn't matter, because you'll never get any of the dashes anyway, because they would be 5th from the right.
RIGHT([oldSSN],4) just gives me the first 3 characters and a dash, e.g. 235-55-7777 becomes 235-
That looks more like the results of a LEFT([oldSSN],4).
Same results... note that this is stored a varchar not integer