thats easy, but you cant get nyatchi on V4. only V3. surprise the shopkeeper(this is for V3) and then type in the teddy bear code and it comes up with rare(that will be the costume for nyatchi). (well, it did for me. otherwise you might have type the code in again and buy it.) click on it when it is in your items(has to be an adult) and your tama will go off the screen for a few seconds and wil come back as nyatchi. click here to see the V3 chart with the costume/rare characters and you will see nyatchi. if you need any more help with this, feel free to PM me.
oh and by the way, if you click on the costume again while you are wearing nyatchi, it will turn into hohotchi. so whenever you put on the costume from that moment, it wil always be hohotchi.