So, currently playing a bit of Assassin's Creed IV: Black Flag which is awesome.
I also got Unity and Syndicate already lined up.
If one would get Rogue as well, were does that fit into the timeline?
And does it really matter anymore since they left Desmond and the numbered games behind?
You'll want to play Rogue after 4 and before Unity, as the former's ending ties directly into the latter's beginning. It's very short, so if you stick to the main missions you'll go through it quickly. Besides, in my opinion it closes the pirate/American trilogy (tetralogy if you coubt Liberation) in a good way, taking elements from all of them, kinda like Revelations did with Altair and Ezio's games. Plus I love that they named many missions after Rush songs and albums (no idea why, guess someone at Ubi was a fan of the Canadian prog rock trio).
Unity is mostly standalone, with only the beginning being connected to Rogue's final moments and a couple of references to the previous entries here and there. Missions are kinda boring, but it looks beautiful. They changed the parkour mechanics, with some additions being nice (parkour down), and some not (having to press an extra button to start climbing, like in the first games).
Syndicate is my favorite of them all (haven't played Origins or Odyssey yet), and it has several connections to 4. Totally worth playing if you like the series. The grappling hook is fantastic, and adds a lot to the series.