Alright this is a bit stupid and random but I thought it was interesting (and perhaps a bit funny) anyway.
If this game is set during the American Revolution which is around 200 years after the events of Assassin's Creed 2 if I remember correctly.
The American Revolution started in 1775, if we say 1775 + 200 we get 1975 (which in theory could be where a theoretical AC 4 could be set).
Looking on Wikipedia's page for 1975 the first entry is the Altair 8800 - Coincidence right?
EDIT: Also we have plenty of Cold War stuff and the Vietnam War in 1975 so there are plenty of events for a Assassin's Creed 4 :)