Yamaha more then likely will release their upper midrange boards before the next "Tyros 5" or whatever they will call it as the next go round is to replace the S910 series.
It doesn't look like any news or rumors are floating around even for that and it is spring already. S910 was realeased in 2009
Like you Scott I am closed to making the decision on a "Tyros 4" as there have been many favorable reviews here and pro reviews elsewhere.
It seems to be the keyboard of choice with so many useful features and one of creativity, composing, musicality,technology.
I have played a Tyros 4 but it never was setup correctly and was patched into 1 speaker at the dealer so my initial impression was not that wonderful;
I don't know why some pro instrument dealers do this, as it doesn't portray this keyboard to the best of its ability when you try it.
You would think Yamaha sales would make it a point to dealers that the better equipment should be hooked up to at least the yamaha MS04's or good quality speakers when you want to try one.
Anyway I wouldn't be holding my breath waiting for a Tyros 5 right now.
(Lets appreciate what the "Tyros 4" can do as the engineering designers cant please everyone and most agree they have done a good job with this keyboard !)