I know this is an old thread but I would like to bring it back up as I think there is still something wrong with how twitter is handling pages with gallery cards that contain less than 4 images.
We tweeted a link to this page which only contained three images and the gallery card did not appear. At the time we only rendered twitter:image0-2 and didn’t include the twitter:image3.
As the documentation says that all four tags are required we pushed a change that will always render all four elements, and if there is less than four images the content attribute would be empty. This appears to be what twitter wants based on documentation.
We then published a link to this page which only contains two images, and the second two image elements were emtpy. The card still didn’t display.
I feel we have now tried both options and neither work so I don’t know what twitter is expecting.