With the 4c I think the best way to describe it is as the headtracking complementing the eyetracking.
Like with the eyeX you'd glance at the HUD and the whole viewport would shift, which is not what you'd want.
The addition of headtracking to the equation allows you to workaround that in that you can configure it so that the viewport hardly shifts on a radar glance, but that's fine because you've got headtracking to signal the large shifts.
Like imagine a scenario where headtracking is the major movement and eyetracking for the fine tuning, well at least that's one way to configure it. I guess because there are now multiple variables involved, there will be multiple options on how you set it up.
With headtracking only, I found in my case it was not fully possible.
It was great looking left/right/down but not for looking up. On tilting your head back it seems like the 4c lost view of my eyes, with head only I could only get as far as the top of the ships front window. Which isn't great as in combat the ability to look up is pretty important. So looking up on the 4c (at least for me) seemed to be essentially eye-tracking only.
