Actually, I've been using my BK-7 more and more. I finally read the manual, ha ha, and have learned how to use it. The BK-7 styles suit me better, although I'll miss the SA voices and vocalizer. Won't miss the weight! For what I do, the T4 is overkill.
I spent some quality time on both the BK-7M and the BK-5 (keyboard) on my last road trip, and I must say the former impressed me both in terms of styles and sounds...in fact, I think it sounds quite a bit more detailed than Roland's flagship G-70, and certainly a far better overall sonic package than the small entry level Korg like the MicroArranger and/or the PA-500.
I can see how the Tyros4 can be overkill for some...I probably would have been content with a PSR-S910, but I got a terrific deal on the Tyros4, and now, I'm glad I did get it after all. The on-board 250 Gig HD has proved very useful, and, since I play instrumentals, the additional SA and SA2 voices are really rich and expressive. The only thing I miss is having on-board speakers.
For accordion, a module is terrific...a buddy of mine still uses a Roland RA-90 for gigging with his "squeeze" and the BK-7M is a bit reminiscent of that overall sound, but, as I said earlier, with more detail and clarity.