This is going to be difficult as many of the manufacturer's have been using tier III's. They've bought surplus engines to help keep the pricing down and they've sold very few tier IV's. Having said that, it will be very hard to find a used tier IV chipper. I feel sorry for you guys in CA having to meet CA's new regulations.
I just picked up a 2007 Morbark 12. I don't know if it is certified yet but I think it might be. I'll know more next week if you want to email me.
I have never seen a used tier 4. They shouldn't be hitting the market for a few more years. The best you can hope for is a certified tier 3 or buy something new.
Our local dealer out in California has some Tier 4i machines in inventory. The chances of finding a used Tier 4i machine will be tough as these units just started being sold within the last few years. If you would like the dealer to contact you, feel free to email me at [email protected].