Definition monotone4 T0 T1 T2 T3 (gf: rel4 T0 T1 T2 T3 -> rel4 T0 T1 T2 T3) :=
  forall x0 x1 x2 x3 r r' (IN: gf r x0 x1 x2 x3) (LE: r <4= r'), gf r' x0 x1 x2 x3.

Theorem paco4_mon: monotone4 (paco4 gf).