Here my 2 cents

I had Tyros2,3,4 and now the 5(I have the 61 keys version), simply the Tyros 5 has the new better DSPs(versus the T4), and make it sound like......well, heavenly.
The T5 is like High Resolution TV vs conventional TV.
There is nothing wrong with the Tyros4, but, there are Many features and new technologies that the T4 does not have.