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