According to the Wikipedia article, it appears the 4GB model and the 250GB model contain the same hardware. However, there is a cosmetic difference - the 250GB model has a glossy finish whereas the 4GB model has a matte finish. Also, the 250GB model comes bundled with a wired headset.

According to the Wikipedia article, it appears the 4GB model and the 250GB model contain the same hardware. However, there was a cosmetic difference - the 250GB model had a glossy finish whereas the 4GB model has a matte finish. They both now have a matte finish (starting in August 2011). Also, the 250GB model comes bundled with a wired headset.