The Story is covered in more depth on Pokémon Go Hub so you should probably head over to this post if you are a hardcore Pokemon Go player in need of the details but the jist of it is 0.79.4 is 0.79.3 and 0.81.1 is likely going through a code review process to find out where the performance issues came from.
Trainers, we’ve halted the rollout of Pokémon GO version 0.81.1 due to performance issues we’ve observed over the past few days. (1/2)— Niantic Support (@NianticHelp) November 7, 2017
While we investigate the issues in 0.81.1, we'll be rolling out Pokémon GO version 0.79.4 with some minor fixes. (2/2) https://t.co/0VeDm3JB6F— Niantic Support (@NianticHelp) November 7, 2017
As noted on the Pokemon Go Hub post new movesets that were introduced have now been lost as they are not in the 0.79.3/4 release.