Take this rumor with a grain of salt, but Spring 2020 was floated as a possible construction start date by this ostensibly German source:
Here is a short summary of the progress #Gigafactory4 since the announcement of Elon Musk on 12.11.2019..
— #Gf4 #Gigafactory4 (@Gf4Tesla) November 27, 2019
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