The deformation with m=4 (a44) is larger than those with m=2. Its typical size is of the order of 10-3 and the sign is mostly negative. However, these small non-axial deformations cannot affect any observables in experimentally detectable ways. Incidentally, it is an interesting problem how a44 develops as the angular momentum increases [HM94].
There is a supplementary comment on the m=4 deformations: For m=2 deformations, the Cartesian-mesh basis has the corresponding symmetry, i.e., it is invariant for the interchange of the x- and the y-axes. On the other hand, for m=4 deformations, the corresponding symmetry is not exactly conserved by the Cartesian mesh because it is not equivalent between the direction along the x-axis and the direction in a line x=y. This anisotropy of the basis may affect the value of a44. Incidentaly, the sign of a44 should not have a meaning when m=2 deformations are vanishing: A shape with a44 and a shape with -a44 should be equivalent when al0 are the same and al2 are zero.