I'd recommend some of the wood kits that are out there, like the Spyderco Dragonfly and CRKT kit. They are a great kit, and it teaches safety. When they can use them the right way, you can get the real thing.
There is a thread with a 4 year old in the Spyderco Forums here. I am waiting to give my sone one (he will be 5) Spyderco also has a Delica trainer that is identical to the real thing but with a dulled blunted blade. I think these would be good to learn knife respect. It think possibly his "first" will be a Swiss Army variety. Not as sexy but amazing fun for a kid.
