I would imagine they're using Arma 4 as the game to build the engine around.
My pc wasn't really powerful enough to play Arma 1 online, so I stuck to the single player stuff and mission editor, like I did with Operation Flashpoint, but I played the shit out of Arma 2. I had to leave Arma 3 untouched unfortunately as my pc was once again too shit tby the time it came out.
I'll be ready for Arma 4 and judging by those shots it's going to look damn good.