[ Diff(x[1], t) = ((-1)*k[1]*k[2]*x[1] + (-1)*k[1]*k[4]*x[1])/k[1],
  Diff(x[2], t) = (1*k[1]*k[2]*x[1] + (-1)*k[1]*k[3]*x[2])/k[1],
  Diff(x[3], t) = (1*k[1]*k[3]*x[2] + 1*k[1]*k[5]*x[4])/k[1],
  Diff(x[4], t) = (1*k[1]*k[4]*x[1] + (-1)*k[1]*k[5]*x[4])/k[1] ]