Hi Toril.
If you're happy with what you already have, just be patient.
The "Special Edition" of T4 is not much more than the "ordinary" T4. 10 Years celebration model, black, limited numbered items produced.
Just wait and see, because all that sudden you will come across a T5 to a reasonable good price...... it's just a matter of time.
Just take a look at Finn, now when Yamaha lowered the Genos prices, more T4 and 5 shows up. I'm pretty sure that pricelevel will lower more during the spring.
(But, myself I got a fair deal swap my T4 for a Genos. That one ment for me is probably still in China, or still at the ocean on the way to Europe.
I was about to add a PA4X-76, and keep T4, but now Korg have to wait. Maybe they have a new model on the way in the near future. Who knows?)