If you haven' been set in you ways for SR2/3 then 4 would be eaiser to pick up. The rules are easy to learn and a bit more uniform, and you won't get flooded with as many extra rules books, as they're not all out yet.
Give you a chance to pace your self in to the game.
If you're looking for a few things to catch up on for details , there's a few books to look at.
Year of the Comet covers the '60-'61 area with some changes in magic.
Blood in the Boardroom goes over the fall of Fuchi and the rise of Novatech
Renraku Arcology: Shutdown covers the rise of Deus, the otaku, Renraku's continuing problems, and keys in to where the metaplot goes next.
System failure covers the '64 crash
Shadows of North America, Europe, and Asia cover all of those areas in the '63 area, and have a lot of setting info. Most of the details can be found on the Timeline explorer her on Dumpshock (when it's back up) in extreme detail, or in the Sixth World Wiki in more over view.