Got new mk4....my focus beam on the 430ex 3 is not firing...it will fire on my 6d's. I did check the menu to indicate that af assist beam is enabled.
Which menu? The mk4 or the flash?
I did both menus...as I said...it fires fine on my 6d's. Now I did try my laso transmitter on the mk4 and that fires the beam no problem.
I suggest you call Canon.
The IR AF focus assist beam will only activate when in One Shot, it will not activate in AIServo.
The IR AF focus assist beam will only activate when the camera can not achieve focus without it.
It seems that it will not fire if you are using the precision focus point...also does not seen to fire if the focus point is not in the center.