EOS 5D Mark IV: Bulb and Auto ISO

When shooting with the EOS 5D Mark IV, the camera can be set to Auto ISO so the ISO speed adjusts rather than the shutter speed or aperture to achieve the correct exposure. If you are shooting in Bulb mode and have Auto ISO set, the ISO speed selected by the camera will be ISO 400. If you wish to use a different ISO speed, then you should select it manually.
Camera settings
EOS 5D Mark IV