I'm going where there is NO mobile service so I tried to download a map:
1. No satellite images
2. Cheap map with no terrain

3. Size limit, appears to allow 65MB but any bigger tells me the "Server" has an issue (The size of the downloaded maps exceeded the server capacity) but I have  an SD CARD with 48GB free so what server?
I have an Galaxy S4 running the latest version of DJI GO 4. Mavic 2 Zoom.

all you have to do to cache images on go4 is zoom in and out around the areas needed to fly when connected to your wifi, there is no image download needed.