I dont think Yamaha is aiming at the pros at all, because in Yamahas vocabullary, pros don't play arrangers, they play workstations. The Tyros 4 is there to make ammateurs like me sound like the pros do...
You are partially right, Bachus, in that the Tyros4 is not specifically "aimed" at "pros"...however, that hasn't stopped many professional players from using the instrument. In fact, many pros also use it's baby brother, the PSR-S910.
Some arranger players, including pros, just happen to like a "user friendly", terrific sounding instrument and easy to use operating system.
Playing an arranger requires a different, but no less important, set of skills than it does to use a "workstation" and those who have honed these abilities to a fine edge, have benefited greatly from having an arranger under their fingers.
Ian