I'm actually back with my 4S as the GS4 was a bit too large for me personally. I am still undecided about where I will go and will likely hang tight and see what happens this fall.
Regarding the screen, I found the screen to have adequate brightness inside, however, I had trouble with it outside, on bright days. I did have mine on manual brightness and just learned from this thread that it actually gets brighter on auto, which is good to know.
I had my old phone on manual brightness as I didn't like the calibration. I use a widget to switch toggle between 3 different brightness levels. My indoor level is hard to see outdoors, so the widget makes it easy to toggle it back to 100% if I happen to walk outside on a bright day first before adjusting the brightness.
My current phone's auto brightness is calibrated in a way that I like. It's nice to not have to manually adjust the brightness if I go outside and back inside.