Hi chony
Very interesting review, with the initial statement being completely opposite to my impression, in that apart from the SA2 voices I found the T3 to be overall a retrograde step after the T2, (1 Step forward, 3 steps back) whereas from what I have heard so far, the T4 is what the T3 should have been, and a true replacement for the T2. (An opinion shared by a lot of users on various forums as well)
Just goes to show how different users interpret the sound and OS of instruments.

There were problems with the processing power of the T3 (Well Documented) which when using some of the premium vocal packs, caused the styles to stutter, and weird sounds to occur, therefore the T3 was already maxed out and was unlikely to be able to be updated further. (The new T4 has significantly increased processing power, (According to Yamaha) and the vocal packs now work sweet as a nut)

I frequent various forums, and the bugs you mention I have not seen elsewhere, and this combined with the fact that you think the T4 is only marginally better than the T3, (Plus Yamahas normally rock solid OS) would lead me to suspect that you have a faulty keyboard.

I would contact a Yamaha service centre to see what they say.

I noticed that a link to this thread has been posted on the YPKO Forum http://www.yamahapkowner.com/forum/index.php so it might be worthwhile keeping an eye on what they come up with.

I hope all your problems get sorted quickly, (I am sure Yamaha will sort it, as soon as they are made aware of it)

Regards

Bill