Outdoor brightness is important to me. I use my Galaxy S4 on a clip on my motorcycle handlebar as a GPS. The S4 is marginal, but it works.
So.....
I did a test with my S4, a coworkers S5, and an LG G3. I set them to max brightness, and put up the same Google Maps screen on them, and took them outside in bright sunlight. The G3 was a little brighter than my S4, and the S5 was even brighter. Very easy to read the map outdoors on the S5. And the more saturated colors makes it even better. The G3 colors were more subdued, which makes the maps harder to read.
THEN, I went to a Best Buy. There, I compared an S5 to a Note 3. The Note 3 was about on equal with the S5 at max brightness, indoors. Then a week later, the BB store got in the Note 4. The Note 4 screen is even brighter then the Note 3, (still indoor at max brightness).
So, in my person subjective test, I'm convinced that the Note 4 will be very easy to read in direct sunlight.
The next question, for me anyway, how much power will it take to light that huge slab at that brightness? My current charging system may not keep up.