A "couple" of years is less than a "few" years. "Couple" can be literally two of something but, more informally, can mean more like 3, 4, or 5 - basically a small amount. However, "few" means more than three AND more than just a "couple" - basically, a small amount that's bigger than a "couple".
Example One:
Person 1: Can I have a couple pieces of candy?
Person 2: Sure, go ahead.
[Person 1 takes 2 or 3 pieces.]
Example Two:
Person 1: Can I have a few pieces of candy?
Person 2: Sure, take what you like.
[Person 1 take 5-7 pieces.]
Hope this clarifies for you. :)