Collecting stamps in Super Mario 3D World + Bowser’s Fury can be rewarding in most cases, but on some levels, rather frustrating. They can often be placed in spots that you will walk right by and never have any inclination to explore. That is true in the castle level in World 4, called Lava Rock Lair. Here is where to find the stamp in the World 4 castle in Super Mario 3D World.
Before you can get this stamp, you need to have a Catsuit power-up equipped. You can either come into the level with one on or in your inventory, or you can get one from the red ring coins. When you have one, proceed through the pipe after the red ring area.
When you get out, on the right side will be a purple box that transports you to a challenge for a green star. However, for the stamp, we need to climb up the wall on the left.
There is a ledge further up that you can land on, then climb up again to find the stamp.