Hi @kleinerber, thanks for your query. As you correctly pointed out, the Tobii 4C Eye Tracker only support officially up to 27″ for optimal use so we cannot recommend that the tracker will work well with your proposed 40″ Screen.
That being said, a few users have found means of creating custom mounts to get around the size requirement by placing the tracker at a distance from the screen:
This procedure however is not one we support although it may suit your needs as you can see from that thread a number of users have been successful in setting up for larger monitors.
Finally, you can still use the eye tracker with a 40″ screen without modification but the area for eye tracking will remain at a maximum of around 27″ as explained in the following article:
Hopefully this information is of some use to you, please let us know if we can be of any further assistance.