Not certain what the OP budget is, but I recommend calling sales and inquire if the tires/wheels are still available as "wholegoods". Typically the price of buying the tire/wheel assembly through wholegoods is considerably cheaper than inquiring about the parts individually.
Theses would be ready to bolt on and the existing set could be sold to recover some of the cost.
Another option would be to consider other manufactures and styles of R4's. I've seen thread patterns that were very deep to shed mud or ones that created less contact pressure.