  We work toward the arithmetic quantum unique ergodicity (AQUE) conjecture for
sequences of Hecke--Maass forms on hyperbolic $4$-manifolds. We show that
limits of such forms can only scar on totally geodesic $3$-submanifolds, and in
fact that all ergodic components of the microlocal lift other than the uniform
measure arise from the uniform measures on these submanifolds.
