Okay, I am an expert on what to get for kids,....not only things that they will enjoy and enjoy for years, but things that will work on their motor, cognitive and social skills. The fisher price little people are wonderful and you can find them in great used condition for half the price on Craigslist Portland. (website) They can play on their own and you can play with them. It teaches social skills, pretend play...depending on the sets you get they can practice caring for babies? animals? My son loves these sets. The large sets also come with a Little People DVD. Your son may or may not be interested in the DVD yet.
Also, my children LOVED the Fisher price Laugh and Learn Learning House. EVERY child that came to my home, no matter how old they were, played with this thing forever! I played with this. This was maybe the best purchase I ever made for my children. I think they still run about $60, but they are worth every penny!
Simple, wooden blocks that they can practice stacking are always a favorite, and focus on their fine motor skills. Look for the colored ones. You can find a box of these for $10.
Something else to start looking into is an easel. I was never really interested in these, but I had an occupational therapist tell me that an easel makes them work against gravity to strengthen the muscles that they will use to write. Your 1 year old may not yet be ready for one, but your four year old definitely is. You can find one that has more than one surface or that is 2 sided, and the great thing about having an older child is that he will model for your baby how to use it. He'll start copying brother right away, even if he really can't do it yet.
All of these toys your older son will have LOTS of fun playing with too (especially the Learning Home), and it can help them play together and share.
Try to stay away from toys that work for them or talk for them. Things like Little Touch Leappads, or activity centers that you sit them in. Mega Saucers and such. Remeber to keep it simple! :) Good luck!!