Well to get an accurate synth sound (Particularly analogue) you either need a software version of it, or a large number of samples of it, (A synth is constantly moving so unless the software does the same or you have a large enough number of samples of it that you can move around) otherwise you are never going to get an accurate sound, and as the T4 has no synthesis on-board and limited sample memory, it will always be at a disadvantage.

On the other hand, if you just want something similar to add to your performance (Which a lot of people do (Hence all the easy play features available on an arranger) then they will most likely be fine. (Remember it what suits you, not somebody else)

For those that have a computer, try FM8, Absynth or Massive to get some real synth sounds. (There are many others, but the above mentioned are reasonably priced)

Details of them can be found here

Bill