I’ve been using a VPN to access Netflix from other countries (licensing deals are extremely different in other countries) and, because Season 4 is avaliable in the UK, Netflix has been using that poster for the show. So I guess that’s the official poster.
Same thing happened with Seasons 2 and 3. Even though the Season 3 poster was just Evil Queen Regina (in her Season 1 Evil Queen outfit) and an apple, it was used as the DVD/Blu-Ray cover just because that was the official poster for the Season.