# File ms6body.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+(ap0+ (jdt+sdt2/4)/3)*dt/2)*dt
      @vel += (ap0+(jdt+(sdt2+cdt3/4)/3)/2)*dt
      @ap3 = @ap2
      @ap2 = @ap1
      @ap1 = ap0
    end
  end