I have a vrisimo shredder and it is designed specifically for the purpose that you describe. It is an orchard quality mower and intended for frequent commercial duty usage. They call it severe duty. Mine is but a small 36" flail but it tops the scales at 820 lbs with the shaft. I believe a 4 footer will weigh in at something like 1150 lbs.
The normal manner in which they are used is in orchards, after pruning the trees, the branches are deposited in a windrow no wider than the machine mower width. The tractor is then driven over the branches and the flail mower using heavy duty hammers shreds them. These mowers are also fitted with rakes at the rear the keeps the material in the mower until the size is reduced to desired levels before being released from mower.
I don't know how you will get it shipped to HI, however. Furthermore, they are hugely expensive to begin with (figure about $7k to $8k for the shredders.
Vrisimo Equipment Company