This questions keeps coming up and I have answered it many times before.
Here we go:
1. Hyperion does not have a license to produce a 68K version of Amiga OS 4.
2. One of the core components of OS 4 is ExecSG which has some PPC ASM in its hardware abstraction layer. Could it be converted to 68K? Sure. Would it be worth the effort? I don't think so.
3. OS 4 is simply too slow on 68K systems (especially the new Intuition and the new Antia-Aliased Fonts) with the exception possibly of top of the line 68060/66 Mhz machines. How many are there of those?
4. It would be a big waste of time and money to maintain two different branches of the same operating system.
Let's face it, the 68K architecture is dead.
It has served us well over all these many, many years but it is time to move on.