Sure that matters, but it's a little bit of both.
Walker and Zach's strengths / weaknesses were a good complementary match for the foursome format today.
In the 'old days' a pairing of Furyk and Toms just wasn't a strong choice for the fourball format where ballstriking (including length) ability is more important than world rank based on stroke play.
I have the MP-69s and love them. when I did my shaft fitting I used MP-4s and loved them as well. The MP-69s are a little bigger. Feel is great. I love them. I hope to have them for a very long time.
if you google "Mizuno iron numbering" you'll get a couple of hits from other golf forums. I didn't see anything obvious in a search here.