I put a #6 blade in a #4 head and it still worked fine, so I guess it's not that much.
You need it, take it. Make sure you have the right head. #4 blades fit #4 head. #2 blades come with a #2 head. Etc. I put a #6 blade in a #4 head and it still worked fine so I guess it's not that big of a deal.