{ diff(x1, t) = (1*k12*k1 + 1*k12*k11*x3 + (-1)*k12*k2*x1*x3*exp((-k3)*x4) + (-1)*k12*k8*x1)/k12,
  diff(x2, t) = (1*k12*k2*x1*x3*exp((-k3)*x4) + (-1)*k12*k4*x2 + (-1)*k12*k8*x2)/k12,
  diff(x3, t) = ((-1)*k12*k11*x3 + 1*k12*k4*x2 + (-1)*k12*(k8 + k9)*x3)/k12,
  diff(x4, t) = (1*k12*k5*x1 + 1*k12*k6*x2 + 1*k12*k7*x3 + (-1)*k12*k10*x4)/k12 }