I kind of find it funny that everyone has their own definition of what is "Amiga". Personally, I think it really is about a period in time when computers were FUN! How often do you get that same feeling you got when you would get home from the store or swap meet with some new collection of floppy disks, and started putting them in the drive and loading them up to see what was crap, and what was awesome, and what was simply there to show your DOS using buddies what they were missing. Unfortunately that particular advantage the Amiga had is simply gone. The 'mainstream' hardware caught up and then surpassed it.
To me now, the Amiga is it's OS and how you can run things in such a limited environment, and they are still impressive. How snappy and responsive the OS can be blows my mind. But it's less impressive when you have a multi-ghz system doing the same thing. So for me, to even think OS4 is really 'Amiga', I'd have to be able to run it on a 68k. Since I can't do that, for me the last real Amiga is 3.9 (and possibly AROS if it gets full 68k working.)