This probably goes without saying but since it happened to because I wasn't paying attention I thought I would share it with others.
Some stores (like a WalMart near me) have the "older" squad packs on the shelves with the "newer" ones. I picked up a pack of Hazmat Zombies last week. When I was at the store again later in the week I noticed both the 6 pack and 4 pack on the shelves. When I got I noticed I had picked up the 4 pack. Today I returned it to the store and got a 6 pack instead (no point settling for 2 less zombies at the same price).
All that to say double check which packs you're getting and which ones are still available on the shelves.