w40_4_neat = subs(w40_4,{a(t),b(t),c(t),d(t),...
                        diff(a(t), t),diff(b(t), t), ...
                        diff(c(t), t),...
                        diff(d(t), t)},...
                        {'a','b','c','d','a_d',...
                        'b_d','c_d','d_d'});