My personal opinion based on the online demos
The Tyros 1 was good, but was outclassed by the Technics KN7000
The Tyros 2 was a ground breaking instrument for the home user, and while I was not enamoured with its CD quality sound, it really was a brilliant model from Yamaha.
The Tyros 3 had some great SA2 sounds but the rest of the sound system was like a compressed MP3 file, and for me basically was 1 step forward and 3 steps back for Yamaha. (The T2 and PSR910 (SA2 excepted) blowing it out of the water) It also had processing problems with some of the voice packs when playing a lot of notes. (T4s processing power has been significantly increased)
The Tyros 4 is to me a true replacement for the T2, and puts right everything that was wrong with T3, while at the same time giving you a hell of a lot more. (Sound quality is head and shoulders above T3)

For me the T3 is to Yamaha what Vista was to Microsoft, a good idea but ultimately very poorly implemented, and just like Yamaha, Microsoft poured a lot of money to bring forward the launch of Windows 7, and was what Vista should have been. (And so consequently is one of the bestselling Microsoft OS ever, whereas Vista was one of the worst)

Taking into account how few upgraded to T3 from T2 compared to those that upgraded from T1 to T2, (There was also a glut of T3 on eBay in the few months after its launch, compared to T2s after its launch) also speaks volumes as to how underwhelming T3 was.

Remember this is purely my PERSONAL opinion listening to the online demos, and talking to users and dealers over the years.

Bill