The D-18T is too big for a 4-engine configuration - it might have worked as an interim engine in a twinjet layout (roughly same thrust as 4 D-30KP, though engine-out performance could be a limitation), but then they'd have had to design the wing to accept both 2 and 4 (WS-20) engine configs. This is doable - A330/340 - but not trivial.