I have been using a viewport of one size or another for a while now and had never noticed any of the world being "cut off" in any of them. I never tested it with a rat per se
but I did test it against background graphics of the game that had lines on it, and was assured that I was seeing the whole world, just slightly higher up on my screen. Maybe I'll have to go back and play with the settings again to double check. One thing I'd like you to expound on is what /viewport you are comparing the 4/3 type to....which type of /viewport supposedly cuts off some of the world? Is it higher or lower? For example I tend to use a /viewport that only cuts off roughly 1/5 of my screen (yup, space hog) which is why I loved Tablets, I can shove everything in the bottom and it all just fits perfectly!
Anyways hehe those are my thoughts.