same place it was before - settings... i am not sure that you can actually program Jap into regular MK4, they use a different hardware/firmware/software in Japan for the Nav... besides, to present the Japanese characters you need more than just the programming - when my MK4 was to be programmed with Cyrillics, the first MK4 unit would "speak" in Russian, display russian maps but would show garbled characters instead of cyrillics - so the company that was doing the rusification for me, brought a different MK4 unit and that one was able to display the correct Latin and Cyrillic characters...

on the other hand, i had been proven wrong many times before, so i wonder what results you managed to achieve...