Phase equilibria among the β-Ti, α-Ti and γ-TiAl phases were examined at 1473 K, in order to reveal the M/Nb ratio dependence of the relative phase stability of β phase in the Ti-Al-Nb-M (M: V, Cr, Mo) quaternary systems. In all of the quaternary systems, the expansion of β phase region toward the lower (Nb+M) content side is observed due to the existence of negative interaction to stabilize the β phase between the third and fourth elements (i.e. Nb-V, Nb-Cr and Nb-Mo). Composition dependence of the interaction is quantified as a function of the M/(Nb+M) ratio at 42 at.% Al. The strong interaction energy existing between the different group elements should be taken into account to design the multi-component TiAl alloys.