A nice way to look at the 4-dimensional rotation matrices SO(4)$SO_4$ is that it's universal cover is isomorphic to S^3 x S^3$S^3 \times S^3$. The map S^3 x S^3 --> SO(4)$S^3 \times S^3 \to SO_4$ is given by left and right quaternionic multiplication by a unit vectors.