I've been watching this, and I love the Arma series. Own and play all, including the original Operation Flashpoint! My understanding, its Arma 3.5.
Arma 4 is incoming, but no one knows when. Bohemia has released this version to tide things over and take feedback from the community. It's come out of beta, now version 1.0, a game to buy. Content is a little light compared to a full blown Arma releases (there is no Editor, fewer units, weapons) but content wise, modders are filling this gap and the maps look great! Everon in this release was the classic Op Flash map.
The kicker for me was no Editor (which will come in Arma 4) but there is something which allows you to place units and have your own war, so I'm debating! I do like the classic setting compared to Arma 3's slightly futuristic setting.