I guess the impact would be very small. The historical mistake was that OS4 was developed at all. AmigaOS was dead. MorphOS came to take its heritage and put it to a new level. Things looked promising. Then OS4 was announced with toatlly unrealistic promises. Users were distracted from MorphOS and the market was split into pieces. That was the mistake (and that's why I am still sour against some certain individuals, my memory is pretty well working). Back then there were still plenty users.
FUD distracted them fromMorphOS/AmigaOS and the user base became snaller and smaller. Without that there had been a realistic chance to establish a healthy niche with MorphOS as AmigaOS successor.
But that ship sailed long, long ago. No more tears to cry about that lost chance.
Today this doesn't matter anymoe. There are virtually no new users. Existing users have made their choices. Some prefer this, some that. Eventually OS4 catched up a bit (MUI4, Odyssey) and it's hard for either of the OSes to really be unique. And while a unification is more than unrealistic the only chance to bring some momentum into this is to broaden the differences. Suggested scenario: OS4 stays ppc and rather conservative/retro. MorphOS sails to new grounds on x64.
According to my observation OS4 seems to be more backward oriented, users pretty much keep the 80/90ies spirit (was a great time, but time moves on!). MorphOS users seem (all on average of course) seem more practical and progressive, using the system primarily for normal computing tasks - which MorphOS can fullfill to some extend, but it becomes harder every day (alone the browser situation gets worse with sites getting more and more demanding and even basic things like the google search page getting issues lately (at least at my setups)). If MorphOs wants to have a future the only solution is to move on. And for that it doesn't matter whether OS4 exists or not.

Short: Years ago MorphOS would have been better off without OS4. And for a few years the coexistence was even good as there was somethig like active competition. But as said this has sailed. MorphOS as "PPC Amiga" is EOL IMHO. And hence, OS4 has no more impact on MorphOS.