[ Diff(x[1], t) = ((-1)*k[1]*x[1]*k[18]/(x[4] + k[18]) + 1*(1 - k[9] - k[15]*k[16]/(x[4] + k[16]))*k[6]*x[1]*(1 - (x[1] + x[2] + x[3] + x[4])/k[5]))/k[21],
  Diff(x[2], t) = (1*k[9]*k[6]*x[1]*(1 - (x[1] + x[2] + x[3] + x[4])/k[5]) + (-1)*k[2]*x[2]*k[19]/(x[4] + k[19]) + 1*(1 - k[13])*k[10]*x[2]*(1 - (x[1] + x[2] + x[3] + x[4])/k[5]))/k[21],
  Diff(x[3], t) = (1*k[13]*k[10]*x[2]*(1 - (x[1] + x[2] + x[3] + x[4])/k[5]) + (-1)*k[3]*x[3]*k[20]/(x[4] + k[20]))/k[21],
  Diff(x[4], t) = (1*k[15]*k[16]/(x[4] + k[16])*k[6]*x[1]*(1 - (x[1] + x[2] + x[3] + x[4])/k[5]) + (-1)*k[17]*x[4])/k[21] ]