Totally go buy the old stuff.
ED4 is very much in its infancy and is taking some time with product development (not a complaint, just a fact). Much of the effort is focused on source books versus scenarios (although I am not privy to their long-term line up, only what they've mentioned so far on these forums or Kickstarter). Even in the case of the source books, they're focusing on developing NEW material versus rehashing old stuff (prior ED publishers have taken considerable, and reasonable, flack for perpetual re-release of old material with minimal alteration). So, good chances that the old material is going to be your best source for a HUGE variety of information and scenarios for years to come, and maybe even throughout the life of ED4.
That said, as I always mention whenever something similar to this comes up - do the community a favor, post any ED4 updates you do to the material so we can help develop the unofficial ED4 catalog and reduce overall effort. For updates, I'm specifically considering stuff like NPC stat blocks, tests, etc. Any updates that people do to source material to bring it in line with the ED4 time line would be hard to consider canon - but who knows, maybe if you post it, Mataxes will look at it, say "yeah, that's good," and wah lah, insta-canon from fan material.
