You COULD also, if you're not using the outlet at all, install a breaker box and run 110 lines out of it. On a 40a 220 line you should be able to run 3-4 20a 110 lines safely. If you don't feel comfortable doing that DIY, hire an electrician.
That sounds asinine. Why the fuck would you do that? Not saying that you can't do it, but why would you? You can run double the rigs at the same wattage on 240 as opposed to 120, because the amp draw is exactly 1/2, but you're ruling out the overload protection and equal distribution per outlet that a pdu will provide ... And better efficiency at 208/220/240.
Efficiency to a degree is valid, yes, though in my house I've noticed an extremely negligible difference when testing antminer PSUs (though they are SUPPOSED to have significant efficiency variance).
Draw? 4x120=480W. 2x240=480W. How are you running double the rigs on the same wattage?
The switches will provide overload protection, and each individual line means tripping one doesn't kill the other three (though I imagine the PDU provides similar protection). 120s allow not having to replace the cords, surge protectors, etc, and allow for more versatile use, spreading machines around, running a spare line to another part of the house, etc. Unless you are specifically using the full capacity (well, 80% of full) a sub panel allows you to get more use out of the line...
EDIT: though to do it properly you'll also want to install a grounding pole, but those are cheap and pretty easy to hammer