I think that, if you are intimately familiar with a piece of kit, small changes will be more exaggerated than to the less familiar listener. To be honest, as a non-Yamaha user, the differences between the T3 and T4 are FAR subtler, from what I have heard so far (including Yamaha's demos) than Ian perceives.
Now, that is NOT to say they aren't there, just that you need a fair amount of familiarity just to hear them.
I can hear differences between E80 and G70 renditions fairly well.
Although it might be difficult to pull off, considering you are living in a place that doesn't seem to cater very well to arranger buyers/users, I can almost be sure, that if you played the Tyros3 side by side with a Tyros4, you
would easily pick up the significant changes in the latter, as I suspect you have a finer tuned ear than most.
Of course, unless your situation gets better, you might not ever get the opportunity.
I have played an E-80 and a G-70, but not side by side,
so I can only say, that you may be right in that, being mostly a Yamaha player/user, that I would find very little differnces, because I did find very little differences
Maybe, if I had played one after the other, I might have noticed more, particularly, if you, or another Roland aficionado, were to point out what to look for (or, in this case, to listen for).
In the Tyros3 vs Tyros4, the most significant thing I can hear, is a distinctly
"sweeter" (my word for "more detailed") sound overall (especially OOTB)...also, most
of the SA2 voices that were carried over, seem to have been tweaked to respond better to the player's technique and/or the SA2 buttons, so actually playing
the instruments in question is important is discerning the differences.
And, that takes us back to your dilemma with no access to new TOTL arrangers...
I hope it improves, my friend.