Found the 3-4 series book in my high school library in 1984. It was most likely written in the 70's. It has been on my mind lately with the new "Wrinkle in Time" coming out. Some similarities, but not the series I remember.
What I remember most is there were 3 children, boy and girl and little brother. Some catalyst causes them to cross over into another world that is a vast desert. They encounter a creature that resembles a dog with really big ears, think it's telepathic. They find other worlds through the desert one. I know there is some kind of conflict, too many years ago for details, but I remember it was a very compelling, meaningful story because I still think of it sometimes.
The last book was very different from the first three in that they were in a futuristic, space setting fighting a bad guy. I remember it was disjointed from the first 3 and I didn't like it as much. I think they were looking for their father which is why I keep picking up "Wrinkle in Time" hoping that's the book, but it's definitely different.
I would love to find it, because this series along with Anne McCaffrey and the book "The Wolf Bell" were the start of a life long love of sci fi and fantasy.