Its been more than a decade, but I remember that Hyperion only had licensed for OS4 not to release OS5 (licensing issue). So it will never reach that version numbering. That is probably one of the reasons they keep releasing updates to 4.1, but they dont change version numbering quickly.
OS4 has stagnated in a kind of silent beta status for about a decade. No groundbreaking new apps & games, just linux ports and an overwhelmingly overpriced hardware scenario that means a very small user base.
Morphos is much more mature than OS4 and takes advantage of second hand macs, which are inexpensive, but its achilles heel is one that it shares with OS4: PPC as a desktop platform is undeniably dead, which means there is no near future for hardware beyond a Powermac G5.
The only viable alternative for OS5 in the long term is Aros. It is open source (unlike others this makes it future proof), runs multiple hardware architectures. Its features and maturity right now, are between OS4 and Morphos. But nothing prevents it going much further.