Because Lionel's gears are back-driveable, two 3-pole motors can jog each other as they're about to stall. So there's a definite gain in pulling power AND smoothness by having two Pullmors instead of one. Just ask GRJ about his Phantom ;-) But four pullmor motors is almost overkill, and a definite amp hog. I'm not sure that one ERR Commander can even handle 4 motors, you might need two Commanders (which doubles the cost of a proposed upgrade.)
As far as a power supply, Right of Way made a 400-watt transformer circa 1990. They're rare and pricey now, but you can easily duplicate it with a set of 24v industrial transformers wired in parallel, their primaries controlled by a 15A "Variac" (toroidal autotransformer.) You'll have to add your own whistle / bell controller, and be sure to use appropriate circuit protection!
I'm not sure if you found Mossback Mike's posts. The biggest issue he cited re: narrow voltage range, is a loco going too fast and leaving the track with more than about 11V on the rails. You have the opposite problem. Once your loco is properly broken in and adjusted, you should be able to enjoy a wide range of speeds from about 7 to 20 or more volts.
Be careful though. Lionel threw some performance away by limiting their Legacy / LC+ electronics to an absolute max of 19V. Heck, the Postwar ZW (which was kind of a quasi standard) went up to 20V, and the Prewar Z to 24 volts! G scale also has an informal standard of 24 volts, and LGB trains with their Buehler motors run very well even without speed control. If Lionel's electronics weren't so limited in terms of headroom, performance gains could be obtained in some locos by swapping in a 24V motor, and supplying up to 24V through the decoder from the rails. I know this, because I've already done it!
