The aftermarket belts are nowhere near as good as stock. Having said that, a belt that goes out of adjustment 4 times in 4000 miles, has other issues. I would suggest that its not being set up properly, maybe the axle is not torqued to specs, the new cam type adjusters can cause issues if the axle is not torqued properly (100 ft/lbs).
Gravel roads are hard on belts, especially if the guards have been removed or modified.
A chain drive eliminates the breakage problem, especially if top quality components are used.
I run a EK 530ZZZ chain, Baker tranny sprocket, Drag steel wheel sprocket. Using Maxima chain wax eliminates 90% of the fling (when used as instructed), chain should last forever.
When I ran a belt, I avoided gravel roads if at all possible, just asking for trouble imo. twinotter