[ Diff(x[1], t) = ((-1)*k[3]*(k[6] + k[7]*k[1]) + 1*k[2]*k[14]*x[2])/k[2],
  Diff(x[2], t) = (1*k[3]*(k[6] + k[7]*k[1]) + (-1)*k[5]*k[8]*x[2]^2/(k[9]^2 + x[2]^2) + 1*k[3]*k[10]*x[3]^2*x[2]^2/((k[11]^2 + x[3]^2)*(k[12]^2 + x[2]^2)) + 1*k[3]*k[13]*x[3] + (-1)*k[2]*k[14]*x[2] + 1*k[3]*k[15]*x[4] + (-1)*k[4]*k[16]*x[2]^2/(k[17]^2 + x[2]^2) + 1*k[3]*k[1]*k[18]*x[4]^2/(k[19]^2 + x[4]^2))/k[3],
  Diff(x[3], t) = (1*k[5]*k[8]*x[2]^2/(k[9]^2 + x[2]^2) + (-1)*k[3]*k[10]*x[3]^2*x[2]^2/((k[11]^2 + x[3]^2)*(k[12]^2 + x[2]^2)) + (-1)*k[3]*k[13]*x[3])/k[5],
  Diff(x[4], t) = ((-1)*k[3]*k[15]*x[4] + 1*k[4]*k[16]*x[2]^2/(k[17]^2 + x[2]^2) + (-1)*k[3]*k[1]*k[18]*x[4]^2/(k[19]^2 + x[4]^2))/k[4] ]