I've pulled an 8-9,000 lb tractor on a heavy trailer. Up steep hills and down them. Not regularly, but on occasion. The V8 can handle it, in acceleration and in braking, without a problem. The only thing is that the trailer that heavy seriously sags the rear end, requiring a height adjustable hitch, jacked up. Otherwise the trailer would be aimed down and could bottom out turning into/out of parking lots or driveways.
I would not hesitate to tow larger trailers for short distances, since it seemed to handle the tractor so well.
As far as safety - the trailer can bounce around the 4runner a bit, so you have to drive smoothly. I've had it up to 60mph only and had no problems. YMMV.