Is this a somewhat square, flat, contiguous 4 acres? What are you doing with the ground? Why not fertilize once per year and just put more on? Reason I ask is have you talked to a local coop or fertilizer dealer? Most all of them have carts for rent and some will have carts that will have either gas powered or ground driven fans so you could pul, them with a small tractor or even a pickup. They usually don’t charge much per acre rent but there is probably a minimum. If you could possibly spread all of your fertilizer at one time, you wouldn’t spend much per year in cart rent and you wouldn’t have to worry about upkeep and storage of your own. On the flip side, I completely understand wanting to own your own piece of equipment and then you can use it whenever and however you want. Just a thought.