If your screen is 125", you’re much more likely to notice a difference with 4K than most will with screens half that size. You could probably live with out, but I don’t think the quality increase would be insignificant.
Just to add another thought to this discussion before we let it rest, another reason I don’t want to change is because this projector is in a completely light controlled room that I can make pitch black in the middle of the day, so I decided to go with an LED projector since it’s reduced brightness wouldn’t really be an issue. (And it isn’t. It looks great) so I could watch it 40 hours a week for 10 years or so without so much as changing the lamp, it’ll probably outlast the projector itself. So I should probably just enjoy the projector as much as I want and then by the time it dies 4k projectors might (hopefully) be odown to around the same price I paid for this 1080p one.