Board Game Arena

There are 2 new stacks appearing in 4 play game at beginning.
Card showing the 4 shapes.
Card showing the 4 colors.
Both stacks show “3” on them.
What are these cards?

Do you mean the Cheshire kitten cards? They're not stacks, they're single cards (and single use) that let you advance 3 spaces whenever you create a straight line with the indicated order of shapes or colours. And they can be added (or not) for any player count.