I'd take a good look at the Wood-Max economy mechanical drive
chipper. It's similar to the Jinma 8", actually it is a Jinma. You can vary the output chip size by varying the knife angle.
I have a Jinma 8" that I bought used for 1000 bucks. It was on Craigslist last year and I have to say it's a helluva
chipper. Having said that I drive mine with a 84 horse pto tractor but it's rated for 16 pto and above. The ability of a
chipper to chip lies in the flywheel weight and the infeed rate and the Jinma/Woodmax has a heavy flywheel with twin knives.
Mine will handle 8" hardwood logs no issue. Your cannot do that of course but I'm sure even with your limited power, once you get the flywheel spinning, it will handle 4" no issue. It's all in how you feed it and with the controlled infeed mechanism, if it starts to slow down, you can release the infeed to allow the reactor to recover, much like commercial chippers that handle whole trees do. The have speed sensing mechanisms that stop the infeed to allow even the big engines to recover.
When I stuff in an 8' log, I have to stagger the infeed kanually, not that my tractot slows down but I installed a twin plate slip clutch on the PTO input so I wouldn't destroy the
chipper in as much as I'm inputting about twice the rated power.
I use the chips as fuel as well and I found that by changing the knife angle, you can alter the chip size.
Just my 2 cents