why the split season of season 4 broken into 2 sets of 10 episodes?

I don't know.

Yeah, the same thing happened to season 5 and 6. Pretty good question why. Probably it has something to do with marketing. Basically they put the cliffhanger at the end of each part 1 of season 4, 5, and 6 so that way they can increase the hype and possible increase in viewers. But it begs another question which is whether it works good this way.