We examined the microstructural characters of L12-type (Al, X, Y)3Ti (X, Y=Mn, Cr, Ag, Fe) phase alloys, with compositions selected from the regions connecting ternary L12-type (Al, X)3Ti and (Al, Y)3Ti single-phase regions in the quaternary phase diagrams using laser microscopy, scanning electron microscopy and electron probe microanalysis. The residual strain, which was induced during milling of the alloys, was measured by examining the X-ray diffraction peak profile of the alloy powders. Most of the (Al, X, Y)3Ti alloys heat-treated at 1450 K for 24 h exhibited the L12 single-phase structure, while a few alloys containing Ag showed two-phase structures consisting of L12 and Ag-rich phases. The lattice parameters of (Al, Mn, Ag)3Ti and (Al, Cr, Ag)3Ti increased remarkably with increasing Ag content, while those of (Al, Mn, Cr)3Ti, (Al, Mn, Fe)3Ti and (Al, Cr, Fe)3Ti decreased gradually with increasing Cr or Fe content. The change of the lattice parameter with the contents of ternary and/or quaternary elements could be explained by the atomic size effect. The size and fraction of porosity, which were formed during heat treatment at 1450 K for 24 h, depended on the composition of the alloys. The changes of the size and fraction can be explained by the Kirkendall effect, irrespective of alloy systems. The increase of the residual strain by variation of the composition in the Al–Ti–Mn–Ag and Al–Ti–Cr–Ag systems may be explained by the fact that the decrease of size and fraction of the porosity increased the residual strain. However, in the Al–Ti–Mn–Cr, Al–Ti–Mn–Fe and Al–Ti–Cr–Fe systems, the change could be explained by the fact that the residual strain increased as the sum of the contents of Al and Ti decreased.