Electronic – At what cable lengths are termination resistors required for RS-485 networks

I would think it would be asking for trouble if you did not use termination and grounding. See Jan Axelson's write up. Also the further your cable extends, the lower the baud rate will be. At 4000' the maximum baud rate is 90kbps. I think using the shield as the ground is okay, but you need to isolate each node with 100 ohm resistors. I have used RS485 for short hops (~100' or so) without using termination or grounds, but I think for K's of feet you would be wise to stick to the rules.

You must provide a common potential reference, so you need at least one pair for simplex data and one wire for common/GND. Just because it's differential transmission doesn't mean you only need two wires.

The power supplies to the nodes should be floating, and the 0V of the floating supply should be connected to the 0V on the cable. That way the node gets referenced to the master's 0V potential. For electromagnetic compatibility reasons, you should put a 1Meg resistor and a 1kV 10nF capacitor between 0V and chassis on each node, unless your nodes are truly isolated and have no exposed metal.

There's nothing wasteful about using Cat5e cable. It is the most ubiquitous type of high-performance balanced data cable you can find. You can buy it in grocery stores, after all.

Sidebar: If your RS-485 receiver chip is isolated or hooked up to a power/data isolator directly, you could do tricks with just using one pair and locally deriving the reference voltage, but that's making things way too complicated when you have the wires available.