Yeah I think the requirement is that you have to have 7 guys on the LOS.
Also, Missouri has had this weird Shotgun formation for a while now, called the "Monster".
I think it has more than 4 plays now (maybe 6 IIRC) but it's still an interesting formation. I don't think I've even seen them run it though.
Then there's the "Swinging Gate" Formation some teams have used on occasion:
