@Lakotaubp @advocatux Thanks!
I followed jamieweb's way to modify ubports-installer(https://www.jamieweb.net/blog/installing-ubports-ubuntu-touch-on-the-meizu-mx4-ubuntu-edition/#fixing-the-failed-remote-unknown-command-error) and got UT installed on my MX4 finally!
This latest method is proved to work and should be topped.
Many thanks to @jamieweb and @AlainW94!