{ diff(x1, t) = ((-1)*k3*(k5 + k6*k1) + 1*k2*k14*x2)/k2,
  diff(x2, t) = (1*k3*(k5 + k6*k1) + (-1)*k4*k7*x2^2/(k8^2 + x2^2) + 1*k3*k9*x3^4*x4^2*x2^4/((k10^4 + x3^4)*(k11^2 + x4^2)*(k12^4 + x2^4)) + 1*k3*k13*x4 + (-1)*k2*k14*x2)/k3,
  diff(x3, t) = (1*k3*k1*k15 + (-1)*k3*k16*x3^2*x2^k19/((k17^2 + x3^2)*(k18^k19 + x2^k19)) + (-1)*k3*k20*x3)/k3,
  diff(x4, t) = (1*k4*k7*x2^2/(k8^2 + x2^2) + (-1)*k3*k9*x3^4*x4^2*x2^4/((k10^4 + x3^4)*(k11^2 + x4^2)*(k12^4 + x2^4)) + (-1)*k3*k13*x4)/k4 }