I don't really know how commercially viable a 5-man or 4-man tour would have been in the 2000s and onward. A significant portion of people that filled the arenas and small stadiums were there for the 3-man era hits. You could see/hear/feel the waning attention of many during In the Cage, Ripples, etc.
What could have worked might have been a 5-man or 4-man selection of shows in particular markets, which might draw many from other locales. If they did say London, Paris, Rome, New York, they might have been able to fill larger venues with fans of the older material. I still don't know how profitable it would have been - a lot of tours are losing money until the final shows.
Anyway, I totally understand why the band chose to have a 3-man reunion once the 5-man option didn't materialize. To do a 4-man reunion would have awkwardness in a variety of scenarios. If they focused on the catalogue up to 1977, Phil would have had to sing a lot of songs that he hasn't sung in a long time and may not be that into anymore (never mind how into them Tony and Mike would be). And skipping any 3-man songs would definitely reduce their overall ticket sales. I also can't see getting Steve to play guitar (or alternatively bass) for Duke's Intro, Domino, Invisible Touch, etc. I suppose they could have done something like what The Eagles did for their History of The Eagles tour: first set with earlier songs and earlier personnel and second set with later songs with a different set of personnel. But that would have had Steve sit out for the second set and would have made him feel like second fiddle for the tour.