The V4 secret characters are Makiko (girl), Nonbiritchi (boy), and Tensaitchi. (boy) To get Makiko have a girl and get her to have 350 kindness points and 100 combined kindness and intelligence points. It's okay if your Tama turns into an adult before it gets these points, just as long as it's not an oldie.
With Nonbiritchi do the same as with Makiko except your Tama has to be a boy and have 350 kindness points.
With Tensaitchi do the same as with Nonbiritchi except your Tama needs 350 intelligence points.
And then of course there is Oyajitchi which you get from mating 2 oldies.
GotchiGirl96