I've seen a 3.73? and a 3.800. The guy that had them wanted stupid money for them so I passed since they were used.

I called Winberg and they can make them in the 3k range. I've seen around 8-10, 3.55-3.58 on eBay. They usually went for a decent price, but at the time I wasn't looking to buy.

I think a 4" is possible but you'd have to open up the cam tunnel for clearance.