I'd like to address this and blame the shitty hardware on my PC. I was able to get 4k@60 working just fine on Windows and Arch with a borrowed GTX 970, but my R9 380 and GTX 660 had issues. I think if I had "4k ready" hardware, I would have had better results. I'm going to pick up a RX 480 when it releases if the benchmarks back up the press releases, and I'll report back again.
A409U seems to have problems of color leaking in gray backgrounds. For example, if i put a picture of thick red line on a gray background, there will be a faint red line extending to the gray background.
This only happens on certain settings. for example, I'm using HDMI from my r9 380 for 3840p@30hz right now and if I use "point to point" display scaling mode, I don't get any issues, but with "wide" display scaling mode, I have color leaking on occasion. I wonder if they'll release a firmware update to resolve this issue sometime soon. (seems like a software issue, rather than hardware)
UHD400 seems to have less dead pixels
Can't speak for UHD400, but my A409U has a single dead pixel about 3 inches right of dead center. It doesn't bother me too much, but it's there.
Just wanted to add my 2c to your explanation.
That said, I think you're right on with your analysis.
If it really is flicker free, then i would buy it over the A409U.
I'm willing to do a flicker test, I just don't know how to do it. If someone can link to a guide or explain how, I'll flicker test my A409U.
While it's true that 40" are big, I'm happy with the pixel density of them and since my eyes aren't perfect, default text size (without scaling) is very easy on the eyes.