My understanding is that the alpha acid content declines with age, but it varies from hop to hop. Some say that, as this happens, other bittering agents become more prominent. Aroma products don't deteriorate as quickly. Still, 4 years old is pushing it a bit, I would say; and have they been refrigerated even if not frozen? I got some hops from Stocks Farm recently, the leaf hops had a best before date of about this time next year and the pellets of the same hop had a two year date. I think it's all a bit vague. I also remember remember that Scott Janish had something interesting to say on the matter. I'll look it up later.