We study the Einstein relation for the diffusivity-mobility ratio (DMR) in ultrathin films ternary and quaternary alloys in the presence of an arbitrarily oriented magnetic field on the basis of a newly formulated carrier energy spectrum. It is found, taking Hg1-x Cdx Te and In1-x Gax Asy P1-ylattice matched to InP as examples, that the DMR increases with increasing surface electron concentration, decreasing film thickness and alloy composition respectively in the quantum limit. The magnetic field enhances the numerical values of the DMR and an experimental method is suggested to determine this ratio in degenerate materials having arbitrary dispersion laws.