I never have luck trying to clean leather, it seems like it is hard to get out oils, grime. Call around and get some quotes for cleaning you never know in a tough economy you might be surprised. You could clean one or two to start, maybe sell one or two to pay for cleaning the others, unless you need all four.
I have heard about a product called Lexol PH that is used by shoe repair stores, you can look it up and read about it.