Four-dimensional space is the highest-dimensional space featuring more than three regular convex figures: 

Two-dimensional: infinitely many regular polygons. Three-dimensional: five regular polyhedra; the five Platonic solids which are the tetrahedron, cube, octahedron, dodecahedron, and icosahedron. Four-dimensional: six regular polychora; the 5-cell, 8-cell or tesseract, 16-cell, 24-cell, 120-cell, and 600-cell. The 24-cell, made of regular octahedra, has no analogue in any other dimension; it is self-dual, with its 24-cell honeycomb dual to the 16-cell honeycomb.
Insects of the superorder Endopterygota, also known as Holometabola, such as butterflies, ants, bees, beetles, fleas, flies, moths, and wasps, undergo holometabolism—complete metamorphism in four stages—from (1) embryo (ovum, egg), to (2) larva (such as grub, caterpillar), then (3) pupa (such as the chrysalis), and finally (4) the imago. In the common ABO blood group system, there are four blood types (A, B, O, AB). Humans have four canines and four wisdom teeth. The cows stomach is divided in four digestive compartments: reticulum, rumen, omasum and abomasum. In chemistry
Valency of carbon (that is basis of life on the Earth) is four. Also because of its tetrahedral crystal bond structure, diamond (one of the natural allotropes of carbon) is the hardest known naturally occurring material. It is also the valence of silicon, whose compounds form the majority of the mass of the Earths crust. The atomic number of beryllium
There are four basic states of matter: solid, liquid, gas, and plasma. In physics
Special relativity and general relativity treat nature as four-dimensional: 3D regular space and one-dimensional time are treated together and called spacetime. Also, any event E has a light cone composed of four zones of possible communication and cause and effect (outside the light cone is strictly incommunicado). There are four fundamental forces (electromagnetism, gravitation, the weak nuclear force, and the strong nuclear force). In statistical mechanics, the four functions inequality is an inequality for four functions on a finite distributive lattice.