Well, my going to my local walmart EVERY DAY has kind of paid off.
I got there today, and 2 vehicles were there. THey weren't there yesterday.
Got another HISS Tank, and the Armadillo/Serpentor set. No VAMP or Trouble Bubble/Ram, which of course are the 2 sets I *really* want.
Just as a curiosity, they were stocking the area when I was there, so I asked the "associate" if they had any more. She told me that she just put them out.. to which I said, "So someone beat me to the other 2" and she said "Nope. Those 2 are the only ones we got in".
So it would seem that the 4 do not come in a single case, rather that the HISS and Armadillo/Serpentor sets come in 1 case, and the VAMP and Trouble Bubble/RAM sets come in another.