Random Technoligies, Dynomax, are a couple that make high flow cats. Don't know if they make anything SPECIFICALLY for the 4.0, but that's no big deal. Just like you don't necessarily need a SPECIFIC model muffler to fit the 4.0, any high flow cat with the right diameter inlet and outlet should do.
I'd only go with the high flow cat if you went with headers and performance muffler also. Otherwise, I don't think you'll see much performance increase for the money you spend. High flow cats are rather expensive. Also, think about this for a minute: If you install headers, high flow cat, and performance muffler, you're putting a high performance exhaust system on a stock performance 4.0 OHV motor. Sure you'll get a little increase, but I don't know if the price or the stock motor justify it. Now if you were to install a performance cam, or get some head work done, or install a blower, etc, then the high performance exhaust system will come into play more. Not trying to talk you out of a performance exhaust system, just trying to point out that a performance exhaust system is not fully utilized with a stock motor. So unless you do some modifications to the motor itself, I'd just go with a good performance catback system, like Flowmaster, Dynomax, etc.
Dollar for dollar, I think the most gain with the least amount of money is a performance cam. But not many people do this, probably because it is a little more involved to perform the installation.
