Overseas auto industry media such as Autoindustriya in the Philippines and Headlightmag in Thailand keep pointing to the upcoming Thailand Motor Expo 2019, where it's expected Mitsubishi will show off what Headlightmag is calling a 'Big Minorchange' - or facelift - for the Mirage. The 2019 Motor Expo is scheduled for November 29 through December 10 in Bangkok, Thailand.
Here's some links to get us started:
Autoindustriya.com: Here's your first (official) look at the 2020 Mirage, Mirage G4
Headlightmag.com: Mitsubishi Mirage / Attrage Big-Minorchange Dynamic Shield
Same article through Google Translate from Thai to English (can be used to translate to other languages.)
Thailand Motor Expo
Same link translated to English
I expect the Motor Expo will be the source of more info on the upcoming Mirage 'Big Minorchange'.