A listing for a Rockman 4 prototype has appeared on Yahoo Auctions Japan. It is one of few protos developed in a Famicom Disk System RAM adapter. No telling just how different it is from the final retail version, but it's not every day we come across something quite like this.
If you're interested on bidding, you'll need to go through a proxy service (Rinkya, for example). Bidding starts at 30,000 Yen (roughly $293.43 US). Source: Yahoo Auctions