Orbital mixing in square planar D4h complexes
All orbitals of the same symmetry will interact (mix) to some degree.

So the ligand SALC will interact with both $\mathrm{a_{1g}}$ orbitals on the metal. The degree of interaction will depend on how close in energy the ligand $\mathrm{a_{1g}}$ is to the $\mathrm{4s}$ or $\mathrm{3d}_{z^2}$ orbitals on the metal.

Generally, the ligand orbitals are closer in energy to the $\mathrm{3d}_{z^2}$ orbital than to the $\mathrm{4s}$.