As a avid fan of the descendants franchise, this movie was very disappointing. It was the first one not choreographed by Kenny Ortega, and boy can you tell.
The songs all sounded the same and most importantly had nothing to do with the plot. They seemed to be awkwardly stuck in instead of seamlessly flowing with the story. Kenny Ortega'a musical theater sensibilities, which shine in the first three movies, are desperately need in this one.
Aside from songs, there are major issues with the plot. The book would have frozen the villains regardless, so the kids going back in time wouldn't have mattered. Ursula 's sister couldn't have pulled the prank at Castle coming.
The character development was very poor too.
My advice? Just watch the first three descendants.