Vasty more money and resources were invested Pace in converting and restructuring RISC OS 5 than Select. It has a modern HAL structure which is essential for future support of modern processors and chipsets. RO4 still has the adhoc non abstracted kernel jumble and the only thing Select did down there was move some of the stuff round and create lots of dynamic areas which are fundementally unsuitable for the 32bit memory map.
Yes Adjust32 was probably converted to 32bit quicker than RO5, because its fundementally the same code as Adjust26 and the oppotunity wasn't taken to make the major architectural improvements. That makes it far more difficult to port to the Iyonix and any other machine, not easier.
All of the high level bells and whistles of the UI and the new APIs from Select can be utilised in any future OS, but the low level structure of RISC OS 4 needs to be consinged to history where it belongs.