Kurt, four wire installations are common for things like stoves, dryers, and jacuzzis because the have both 220 and 110 devices in them. For your compressor and welder, look at the plug; if it only has three blades on it, it only needs the two hot legs and the ground, no neutral. You can run a neutral, but you would just leave it capped off and stuffed in the back of the box, not connected to the receptacle. Save the wire, it's probably not necessary. Now, if you want to plan for future 110 load out there, or even provide for it now, run the four wire to the sub-panel, and then just the three wire legs for the welder and compressor from there. And, you can install a ground rod, but in the desert, it is pretty ineffective, and not called for in the electrical code for the type of install you're doing; again, save the money, and the work - putting down an eight foot ground rod in this ground is usually an exercise in frustration at the least, and futility at worst. That contiuous ground wire between devices and panels is a far better safety means. Yep, I am a "sparky", "wire bender", "lightning wrangler", and other sorts of names for electricians; been retired for a good while, but like my sig. says: "Not entirely dead - yet". The offer still stands.