[ Diff(x[1], t) = (1*k[8]*k[1]*x[1]^(4/5) + (-1)*k[8]*k[2]*x[2]*x[1]/(1 + x[2]/k[3] + x[1]/k[3]))/k[8],
  Diff(x[2], t) = (1*k[7]*x[3] + (-1)*k[8]*k[4]*x[2])/k[8],
  Diff(x[3], t) = ((-1)*k[7]*x[3] + 1*k[5]*x[1]/(10000000 + x[1])*x[4] + 1*k[9]*k[6]*x[3])/k[9],
  Diff(x[4], t) = (-1)*k[5]*x[1]/(10000000 + x[1])*x[4]/k[9] ]