Euler's formula states that for any polyhedron, the number of vertices (V), edges (E), and faces (F) are related by the equation V - E + F = 2.
In this case, the number of vertices is 4 and the number of edges is 6, so we can substitute these values into the equation to find the number of faces:
4 - 6 + F = 2
F = 2 - 4 + 6
F = 4
Therefore, the number of faces in the regular polyhedron is 4.