All good. you were kinda right. It is 5 years since SR4... Sorta
To answer the original question...
Anything that's not rules is still applicable, in it's way. It's history, at the very least.
When I'm writing Seattle, for example, I utilize all three of the Seattle sourcebooks (From 1st, 3rd, and 4th editions). Now that I'm doing Chicago for Season 5 of Missions, I make sure to have Neo-Anarchists Guide to North America (1st ed, which has a small section on Chicago), Bug City (2nd ed), Target: UCAS (Which has a large chapter that follows up on Bug City events 3 years later), and Feral Cities (4th ed, and has a large chapter on 2070's Chicago).
If you're going to be using a location or a setting or a theme, all the books can be useful, because while they will double up on some information, each product often still has unique information to it as well that can be useful and can offer inspiration.
And even older rulebooks can still be outstanding from a perspective and fluff standpoint. The 1st and 2nd ed gear books, for example, all had plenty of Shadowtalk in them giving insight into the items they're discussing, plus adding some excellent reading and flavor. Awakenings and the 1st or 2nd ed grimoire are still, IMO, must-reads if you're playing a magically active character.
You don't NEED anything but the current edition. And the older books will have lesser or greater value depending on what you're doing. But to me, it's like saying "Is World War II worth reading about? It's 80 years ago, who cares what happened?" It may be 80 years ago, but it's had a direct impact on how the world was shaped, so it's still worth knowing what happened.
Bull
