I need help in identifying this piece so that I can search for it on BrickLink. It is a platform for minifigures. It comes with the "mystery minifigure".
You should be able to find this piece by looking up any of the collectible minifigures on Bricklink and looking at the set inventory.
This piece is the Tile, Modified 3 x 4 with 4 Studs in Center (88646).
It is classified on Bricklink under the category 'Tile, Modified'.