Because it was a smallish print run, it sold out quickly and those of us who subscribed or bought it at the time are unlikely to sell because of course we're mining enthusiasts! Hence, the only time I've ever seen copies up for sale is as mentioned above, when PDMHS get left the libraries of deceased members we sell on anything we've already got copies of so there have been one or two copies of vol.4 come through in recent years, but I suspect they'll have been snapped up pretty quickly.