KY-Clay
Here you go. https://www.amazon.com/Mount-MI-484C-Articulating-Premium-Capacity/dp/B0052HT3UO?psc=1&SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-osx-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B0052HT3UO
?
Steven F has the right idea. Four feet is way too long to extend a TV mount, even if they make them. That's a lot of leverage on the anchor points.
You can mount a board, perhaps 2x6 between the two corners, forming a triangle, and attach to that. The TV then extend safely from the 2x6.
Or, find a stud/joist in the ceiling, hang a board from that, then brace it into the corner.
STEVEN F
Are you allowed top mount a board diagonally between studs in each wall?
If you are, you can attach ANY TV wall mount to the board, and the TV will hide the board.