{ diff(x1, t) = (1*k1*k2/(k2 + k3*x3^k4) + (-1)*k1*k5*x1)/k1,
  diff(x2, t) = (1*k1*(k6 + k7*(x2*x4)^k8)/(k6 + k9*x1^k8 + k7*(x2*x4)^k8) + (-1)*k1*k10*x2)/k1,
  diff(x3, t) = (1*k1*k11/(k11 + k12*x1^k13) + (-1)*k1*k14*x3)/k1,
  diff(x4, t) = (1*k1*(k15 + k16*(x2*x4)^k17)/(k15 + k18*x3^k17 + k16*(x2*x4)^k17) + (-1)*k1*k19*x4)/k1 }