The part of the DNA that is know to fail, is the front half. There are replacement wishbones for them, which essentially fixes the weak point, and improves the looks. This brings the price of the DNA up to the $700-$1000 range. Other than that, you're likely to spend more on a new springer, and not likely to find a 4 under vintage springer, since most were used on stretched choppers.
If your trying to spend $500 or less, its gonna be nearly impossible to find a quality new springer. Btw, 4 under is only about 2 inches shorter than a stock xs650 front end.