It is certainly worth considering replacing older parts. I bought my current bike used a little over one year ago. When I got it, I knew the derailleurs were somewhat old and worn. Just a couple weeks ago, I finally decided to get them replaced. It made a world of difference. Everything about the bike feels better. Anyways, 3 or 4 years (depending on how much you ride) is probably long enough that you should look into replacing some pieces of your bike.