When to use four wheel drive?

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I've never taken my tractor out of 4WD since 2013. Well I have, but immediately put it back in. I don't run on asphalt, always on gravel, grass or snow. No issues and tires are all wore almost exactly the same checked with a tread depth gauge. The front treads were 1/32 less than the rears on my R4 tires.