This is what it says in the T4 Manual below:

Make sure your Tyros4 meets the voltage requirement for the country or region in which it is being used.

I can only think that Yamaha make KB for each voltage, there is no sign of a dual voltage, but please look on the back of the KB and the plug on the end of the lead might give some indication, the label will without doubt tell you,it may be also a case of a person going to another country and buying a KB and taking it home, like Skude did with his Audya, the Audya would work because its multi voltage and freq. LOOK AT THE LABEL PLEASE OR BOOM!

Here is the T4 warning

Make sure your Tyros4 is rated for the AC voltage
supplied in the area in which it is to be used (as
listed on the bottom). Connecting the unit to the
wrong AC supply can cause serious damage to the
internal circuitry and may even pose a shock hazard

Tony