I purchased this very comfortable seat, second hand from a breaker, for my ST2 and removed it, after about 4 years use, when the bike was sold. As will be seen from the photos, there are no marks or tears, it is in used, but very good, condition. It may fit a ST4, as I believe that the top of the frame, and mounting points are the same, but have not tried it on one of those bikes.
eBay