Thanks, Tim,
An 8x8 with a 1" peg, 1.5" thru mortise and 1" housing and a 3' kneewall still doesn't quite make it with an actual f of 1029lbs/sq.in. and an allowable of 1000 (note this is higher than any grade of Eastern White Pine, but hemlock might get you over the hump). One thing that Jack might have considered with the 4' limit was the load range; if there was no snow load then the cantilever can be longer. As with any design, local conditions need to be factored in. There are safety factors built in to these calcs, as well.