Ceiling in Pole Barn
before I would go that route,,,would metal work to bridge that 4' gap,,,like metal siding? They even make metal,, thinner,, special for ceilings. you can afford a lot of metal for the cost of cutting the spans to 2' by adding framing.
People blow insulation on that all the time,,,altho a vapor barrier might be a good option also make sure the roof doesnt leak cause it wont be detectable so fast,,maybe leave access for regular checks or storage so you look every so often I had a pole barn once,,,had a terrible time keeping the ridge seam from leaking!! (fwiw)