[ Diff(x[1], t) = 0,
  Diff(x[2], t) = 0,
  Diff(x[3], t) = (2*k[2]*k[6]*x[4] + (-1)*k[3]*x[3]^2 + (-1)*k[4]*x[3]*x[4] + (-1)*k[5]*x[3])/k[1],
  Diff(x[4], t) = ((-1)*k[2]*k[6]*x[4] + 1*k[3]*x[3]^2)/k[1] ]