Sometimes an old fashioned
shovel beats out a fancy roto tiller.
Big, thick, Poke root extraction is a task that
needs someone to invent a solution for in the form of a tiller like
machine. Maybe it will somehow use a small, flexible auger to chase
down and grind up any unwanted roots within a 4 foot deep perimeter?
Hopefully it'll be easier
to start up than our tiller was last year.
I'm surprised you didn't start talking about dynamite again when you saw that six inch deep root that ran several feet into the earth.
(But hold the dynamite )
Build a tripod (wooden or metal beams) over the stubborn root, get a slipknot over the root and use your 4 ton winch to pull it straight up.