You asked for opinion - here is my take on the various O gauge K4s locomotives you mention. (at one time or another I've owned all but the Max Gray K4s). I won't comment on early domestic models as IMO they don't have the fine detail or 1/4" scale dimensions I'm looking for.
KTM K4s imported by Max Gray: Oversize diameter boiler, open frame motor, weak gearbox, crude castings, so so runner.
KTM K4s imported by US Hobbies: Oversize diameter boiler (the smokebox diameter matches that of a M1). Has an incorrect taper in first course behind the smokebox. Open frame motor, OK gearbox, better castings than the MG model - but not up to today's standards. Replacing the headlight and bell are good first step upgrades. I replaced the open frame motor with a Pittman and for many years it was a solid runner my layout.
Williams & Weaver K4's by Samhongsa: Correct diameter boiler but stretched in length to accommodate 3 rail flanges. The pilot hangs far forward of the smokebox front. Drivers are slightly undersized - again to accommodate the three-rail version which shares the same chassis. On the plus side the model was solidly built and had better castings than the KTM models. IMO the stretched boiler unacceptably compromises the look of the model when compared with prototype photos.
PSC K4s : Correct dimensions capture the look of a K4s in 1/4" scale. Nice lost wax brass details. The downside was a compromised drive train setup featuring a small low torque 9.5v Pittman motor. I tossed the spur gear tower drive and retained the original gearbox. I replaced the motor with a larger 12v Pittman. I have several PSC K4's that I've up detailed to represent specific numbered locomotives and they've been good runners for 25+ years. The first thing to replace is the horribly mis-shaped Keystone number plate.
Sunset High Iron K4s: correct boiler dimensions, weak solder assembly, poor looking running boards due to way piping hangers attach to the boards. The model's quality of construction is not up to the standards we've come to expect from Sunset 3rd Rail.
Kohs K4's: The most highly detailed and best-looking O scale K4s ever done - but IMO better suited for a display case than on an operations-oriented railroad. Handling requires special care, especially the unique drawbar coupling between the engine and tender. The model has a second motor buried deep in the boiler to move the reverse gear! Due to the reverse gear linkage and numerous pipes and fittings linking the boiler and chassis a complex process is required to remove the boiler for routine maintenance (and decoder installation). Shorts due to the close clearance between brake rigging and insulated wheels were a recurring problem. As much as I admire the detail and look of the Kohs K4s I sold the ones I owned in favor of upgrading PSC K4's.
If your friend is not a PRR rivet counter a USH K4 or Williams/Weaver K4s would be a reasonable choice. I went with the PSC K4's as I like the dimensional accuracy and level of detail on the model. The mechanism upgrades took care and effort but did not require machinist skills.
