[ Eq(Derivative(x1, t), (1*k18*k1*x2*x1*(1 - x1/k2) + (-1)*k18*k5*x1 + (-1)*k18*k3*x3*x1)/k18),
  Eq(Derivative(x2, t), ((-1)*k18*k10*x2 + (-1)*k18*k9*x1*x2 + (-1)*k18*(k13 + k7*(k8 + x3))*x2 + 1*k18*k10*x4)/k18),
  Eq(Derivative(x3, t), (1*k18*k7*(k8 + x3)*x2 + 1*k18*k4*x1*x3 + (-1)*k18*k6*x3 + (-1)*k18*k14*x1*x3)/k18),
  Eq(Derivative(x4, t), (1*k18*k10*x2 + 1*k18*Piecewise((k12*sin(6*pi*t), Lt(k11, k12*sin(6*pi*t))), (k11, True)) + (-1)*k18*k13*x4 + (-1)*k18*k10*x4)/k18) ]