Oh, one more tip.

No, you don't have to play AC3 to 'get it.' You really don't. There's one big tie-in, and one very very extremely subtle tie-in, and for the rest of the time you can ignore everything, secure in the knowledge that 'AMERICA HAPPENED,' and really, it was all because of an assassin, but really, it was all because Desmond. 

If you're curious about the tie-ins, though, here they are in hiders. Shouldn't spoil anything at all.

[hider=the tie-ins from AC3]
(subtle) Your base of operations in 3 was a plantation/manor/town-type place outside of Boston called 'the Davenport Homestead.' This is insignificantly related to the subtext behind an absent character's story -- yes, it's THAT subtle, I'm not making that up.

(obvious) During AC3, if you manage to suffer through it, you'll (frequently) encounter a very interesting character named Haytham. He's hands-down the best part of that whole experience, and he's connected to AC4 as well, albeit in an extremely limited capacity.[/hider]