tl;dr warning
I imagine a fair number of users would be genuinely interested in OS 4 if support for the PPC Mac's was there. The cost of hardware for OS 4.1x in the form of anything still or soon to be manufactured (SAM, x1000) is a massive stumbling block. I don't ever see it happening, unfortunately. I'd love to be able to run Morph and OS4 on my PPC Mac, and vice versa on my SAM 440ep. Both are real nice OS's to use, and both HW platforms are quite nice despite the fact the modern computer community has passed them both by, spec wise in relation to the atypical Ghz wars on the PC side. Having a dual boot Morph/OS 4 rig would be a real treat for the community. I'd gladly invest in OS 4 for PPC Mac's.
I say that as a guy that was likely one of the first here to buy a SAM 440ep/OS 4, and I don't regret buying it at all even though I am fully aware most people find them excessively expensive for what they pack spec wise, on paper. Real hard to justify in dollars and cents terms.
In the grand scheme of things, adoption of OS 4 has stagnated due to expensive, specific hardware. This relatively low number of users due to said expensive HW has also caused a stagnantion in software development (see the Morph OWB vs. OS4 Timberwolf/Firefox debate).
It is very hard for me to recommend a SAM/OS4 system to someone looking for a "taste of a modern Amiga" for $1000 full built when a Mac Mini and MorphOS can be had for a couple hundred bucks, tops. MorphOS is a very nice OS to use on commodity PPC Mac's. Niche market HW like the x1000 and SAM's won't ever come down much, price wise. Limited run specialty boards are damned expensive to make, where as Mac's are very easy to come by for dirt cheap.
I use OS4 and MorphOS, and enjoy both greatly - but bang for buck goes to MorphOS by a wide margin. What you choose to use merely depends on your preference and your budget, to me. I have not regged MorphOS yet, since I would like to pick up a higher end Mac than this old basemodel Mini and licenses are not easily transferable, but will do so once I get a PPC Mac I consider a "keeper".
Just my 2 bits, there's room for everyone really - but there's no denying that OS4 would be used a heck of a lot more by the community if it ran on the old "surplus" PPC Mac's.