The most likely cause is flash sync limitations combined with Exposure Safety Shift. In Av mode, if the camera needs a shutter speed that isn’t possible with the current flash sync setting, Safety Shift can automatically change exposure settings to make the shot possible. That can result in the camera stopping down from your selected f/4 to f/5.6, f/6.3, f/7.1, or f/8.

If your flash was on normal first-curtain sync, the camera would be limited to the standard sync speed. Enabling high-speed sync removes that shutter-speed restriction, which matches the accepted explanation from the discussion.

IS does not change aperture. And your 24-105mm f/4L is a constant-aperture lens, so zooming alone should not make it become f/5.6 or smaller. The answer suggesting this is just an “effective” aperture change from zooming does not apply here.

So: check your flash sync mode, and remember that with Exposure Safety Shift enabled, the camera may override your chosen aperture in some situations.