A few weeks ago a piece of Avengers 4 promotional artwork leaked, and today more images from that piece have leaked online confirming that it was legitimate promotional art for next year’s movie.
When that original art leaked, there were people online who refused to believe it was real, but these images show some of the same art that piece was made up of. The biggest thing in these images are the new ones of Captain Marvel. The previous promo art had her in one pose, but there are three images of Captain Marvel included here.
— Aj Designs (@ajdesigns0220) June 28, 2018
These appear to be PDFs of character key art from merchandise sources. For example, if you look closely at the Thanos image you can see (in the corners) what looks like toy packaging mock-ups. So this stuff is obviously leaking from a licensee.