Another forum post had me re-examining my 6k and 1k/3k screens.
Because of the way LCD screens are made, they have optimum viewing angles. The 6k screen appears to be optimized for portrait use, with the widest viewing angle available across the shortest dimension of the screen (left - right).
Well, the 3.7" screens are optimized the same way - across the short side of the screen, which is up-down in the most used clamshell mode. Once you put them in the same orientation, the screens have nearly identical optical characteristics and viewing angles. In fact, the 3.7" is retains better contrast than the 4" at certain extreme angles, not that it matters that much.
I believe that Sharp should have optimized the 3.7" screen for landscape mode on the clamshells. This is probably where most of the dissatisfaction with them comes from.
As for brightness, the 3.7" screens appear brighter to me, not the 4". This is comparing a 1 yr old 3k/6mo old 1k to a ~2yr old 6k so it may just be an issue with the aging backlight.