There are six regular convex polytopes in 4D, which are analogous to the five Platonic solids in 3D. This is the sixth, the hypericosahedron, with 600 tetrahedral cells.
This was the hardest of this group to make a printable model of. For a Schlegel diagram one would need quite a large size to allow the amount of interior complexity required, and it gets difficult to build as well as expensive, so I used this face-first projection suggested by Henry Cohn. Some of the tetrahedral are collapsed and become planar, but on the plus side the complexity is on the outside where you can see it!