If all you are wanting is a +4, get the venforcer extension. If you are wanting a +6 or bigger have one made. Get with Gauge or Venforcer, they will make you an extended swingarm, they will make you a new shock mount on it also. But I highly recommend the extension, they are a very nice piece.
Ven can weld you up a +6 or bigger :tup: I am sure him and Gauge both could fix you up a +4 though in the welded swinger if you wanted one of them. I think they charge around $500-600 to do it. (just guessing)