A 42" screen 4 feet away will only appear the same size as a 21" monitor 2 feet away. Remember its all about view distance. Any distance at least 1.2x screen width is usually considered acceptable but if you want the screen further away for being able to show more people thats fine. It will just diminish the perceived size.

If you use it frequently you might strain your eyes though, focusing between near and far all the time. If you start to get a headache you could just try moving it closer.