4.5" on my Sensation, Desire HD, HD2 is not hard to imagine. Have you used any of those phones? It sounds like a ridiculous size but honestly, it can be done using the same dimensions as the HD2-easily. Just extend the screen down the bottom after removing the physical buttons. In fact, carriers waste real estate all of the time. The G2x for example. Same lenght as y HD2, yet the screen is 4".
Personally, I would love to see this in person. I havent checked out AT&T's 4.5" Samsung yet. The only reservations that I have is, a screen that size needs to pack the right amount of pixels. WVGA looks wonderful on 4" screens and lower. After comparing the HD2, Desire HD and Sensation, qHD is the way to go.
I remember my Touch Diamond having a VGA display on such a small screen (2.8") and it blew me away. The reality is, the small screen packed a high number of pixels. Imagine a 3.8" or 4" screen packing VGA resolution? Not so great. Idk, guess we will have to see in person.