iTunes version or Amazon version?

I'd love to buy the entire series as a digital download, but it appears I can only buy one season at a time. I'm in the U.S. (hanging head in embarrassment), comparing prices on Amazon and iTunes. The prices are the same except for season 4. For some reason, Amazon wants $14.99 while iTunes is $9.99. Adding to the confusion, the entire set is available if I buy the DVD, which would also be cheaper, and the DVD comes with extras.

So, does anyone know if the digital download from Amazon for season 4 contains something beyond the iTunes version?

Or, are the DVD extras worth getting the DVD version?

Thank you, from one IT Crowd obsessed fan to others