Occasionally you hear about someone upgrading SAG products from ancient versions to current, but it’s rare, so I wouldn’t expect to hear many experiences.
Check the Release Notes to see if it is possible to upgrade directly from Adabas 3 to 6. An intermediate conversion may be necessary.
It’s been too long to remember specifics of the Natural 4-5 upgrade. I would install version 8 and perform a CATALL CHECK to see what syntax errors pop up. You may even be able to execute your v4 applications without a CATALL.
Regarding AIX, I have a bit of experience with TL updates, but not version upgrades. SAG has always been very good at minimizing the effects of OS upgrades, so I would expect your AIX upgrade to be transparent to Adabas and Natural.