Zogman, if you're still looking for a wood chipper, keep an eye on Ritchie Bros auctions. I've seen the 4" go for $550-850 USD, up here anywhere from $950-2200 CDN. I bought one last fall but haven't found the time to try it yet, soon though. They look like a wallingstien knock off, but brand-new in a steel shipping cage. Once in a while some get dropped and are banged up, look close before bidding.
As far as will your tractor run it, everyone here is full of good advice. Using the chipper at half capacity (2" wood) and feeding by ear (engine or flywheel slowing), I don't think you'd have a problem. It's no different than guys in snow country running a bigger snowblower on a small under rated tractor, they can do it as long as it's not too deep, or not taking full width. Listen to the engine, it'll tell you if it can handle it!
As an extra precaution, I agree changing the shear bolt is a very good idea.