-- Lean 4 proof step from Mathlib.Algebra.Category.ModuleCat.Presheaf.Colimits (goal)
Goal:
C : Type u₁
inst : Category.{v₁, u₁} C
R : CategoryTheory.Functor.{v₁, u, u₁, u + 1} Cᵒᵖ RingCat
J : Type u₂
inst_1 : Category.{v₂, u₂} J
F : J ⥤ PresheafOfModules.{v, v₁, u₁, u} R
inst_2 :
  ∀ {X Y : Cᵒᵖ} (f : X ⟶ Y),
    PreservesColimit (F ⋙ evaluation R Y) (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f)))
inst_3 : ∀ (X : Cᵒᵖ), HasColimit (F ⋙ evaluation R X)
X Y Z : Cᵒᵖ
f : X ⟶ Y
g : Y ⟶ Z
j : J
⊢ Eq (α :=
    (F.obj j).obj X ⟶
      Functor.obj (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g)))) (colimit (F ⋙ evaluation R Z)))
    ((F.obj j).map (f ≫ g) ≫
      CategoryTheory.Functor.map (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g))))
        (colimit.ι (F ⋙ evaluation R Z) j))
    ((F.whiskerLeft (restriction R f)).app j ≫
      CategoryStruct.comp (Y :=
        (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).obj (colimit (F ⋙ evaluation R Y)))
        ((ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).map (colimit.ι (F ⋙ evaluation R Y) j))
        ((ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).map
            (CategoryStruct.comp (Y :=
              colimit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))))
              (colimMap (F.whiskerLeft (restriction R g)) :
                colimit (F ⋙ evaluation R Y) ⟶
                  colimit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))))
              (Iso.inv
                  (preservesColimitIso (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) (F ⋙ evaluation R Z)) :
                colimit ((F ⋙ evaluation R Z) ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) ⟶
                  (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))).obj (colimit (F ⋙ evaluation R Z)))) ≫
          Iso.inv
            (ModuleCat.restrictScalarsComp'App (RingCat.Hom.hom (R.map f)) (RingCat.Hom.hom (R.map g))
              (RingCat.Hom.hom (R.map (f ≫ g))) ⋯ (colimit (F ⋙ evaluation R Z)))))
have rw : Eq (α :=
  (F.obj j).obj X ⟶
    Functor.obj (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g)))) (colimit (F ⋙ evaluation R Z)))
  ((F.obj j).map (f ≫ g) ≫
    CategoryTheory.Functor.map (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map (f ≫ g))))
      (colimit.ι (F ⋙ evaluation R Z) j))
  ((F.whiskerLeft (restriction R f)).app j ≫
    (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map f))).map
        ((F.whiskerLeft (restriction R g)).app j ≫
          CategoryStruct.comp (Y :=
            colimit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))))
            (colimit.ι (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) j :
              (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))).obj j ⟶
                colimit (F ⋙ evaluation R Z ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))))
            (Iso.inv
                (preservesColimitIso (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) (F ⋙ evaluation R Z)) :
              colimit ((F ⋙ evaluation R Z) ⋙ ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))) ⟶
                (ModuleCat.restrictScalars (RingCat.Hom.hom (R.map g))).obj (colimit (F ⋙ evaluation R Z)))) ≫
      Iso.inv
        (ModuleCat.restrictScalarsComp'App (RingCat.Hom.hom (R.map f)) (RingCat.Hom.hom (R.map g))
          (RingCat.Hom.hom (R.map (f ≫ g))) sorry (colimit (F ⋙ evaluation R Z))))
Tactic: rw [← Functor.map_comp_assoc, ι_colimMap_assoc]