# File ms4body_old.rb, line 110
  def ms4(dt)
    if @nsteps == 0
      @ap3 = acc
      rk4(dt)
    elsif @nsteps == 1
      @ap2 = acc
      rk4(dt)
    elsif @nsteps == 2
      @ap1 = acc
      rk4(dt)
    else
      ap0 = acc
      jdt = ap0*(11.0/6.0) - @ap1*3 + @ap2*1.5 - @ap3/3.0
      sdt2 = ap0*2 - @ap1*5 + @ap2*4 - @ap3
      cdt3 = ap0 - @ap1*3 + @ap2*3 - @ap3
      @pos += vel*dt + (ap0/2.0 + jdt/6.0 + sdt2/24.0)*dt*dt
      @vel += ap0*dt + (jdt/2.0 + sdt2/6.0 + cdt3/24.0)*dt
      @ap3 = @ap2
      @ap2 = @ap1
      @ap1 = ap0
    end
  end