The 4 cylinder engines are a lot less common. The 2 cylinder engines are also used with one of the small generators, so they're a lot easier to come by.
I suspect SS classifieds, G503 mule forum, eBay, etc are going to end up being your best bet.
I would talk to Emery as well. SS user Charlie is a mule guru and may have a line on one and SS user Austringer is very 4 cylinder knowledgeable and may have a line.