[ Diff(x[1], t) = (-1)*k[4]*k[1]*x[2]*x[1]/k[4],
  Diff(x[2], t) = (1*k[4]*k[1]*x[2]*x[1] + (-1)*k[4]*k[3]*x[2] + (-1)*k[4]*k[2]*x[2])/k[4],
  Diff(x[3], t) = 1*k[4]*k[3]*x[2]/k[4],
  Diff(x[4], t) = 1*k[4]*k[2]*x[2]/k[4] ]