Pacman install fails with 404

error: failed retrieving file 'X' from Y : The requested URL returned error: 404

Replace /etc/pacman.d/mirrorlist with a new file generated at https://archlinux.org/mirrorlist/.
Then run sudo pacman -Syyu to update the package database with the new mirrorlist. Make sure to use 2 y's, to force the new changes.

Explanation

Your mirrorlist is outdated, possibly due to the recent pacman git migration.