Press the menu button and go to the custom functions menu (I believe it is the fifth tab on the 5D4), change ISO speed setting increment to 1/3 stop. Now you can set ISO 320.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
