I tried to roll back from Emui 5 to Emui 4.1 last monday, following the official guide to the letter. All went okay until the last step, after trying to install the 4.1 ended up with my phone in boot loop, it wouldn't run the os, but didn't stay powered off either.
Managed to enter eRecovery mode, and the phone told me "data partition was damaged seriously!". It offered to download a recovery file, but couldn't establish wifi connection. I took it to a shop, and few days later received a whole new phone.
Super lucky that this replacement is "old" enough to still have Emui 4.1, now I'm doing everything in my power so it doesn't update to Emui 5, not anytime soon anyway.
Yeah. Rollback from 5 to 4.1? Don't freaking do it.