Like with HD before it, there's a lot of smoke and mirrors when it comes down to specifics. Panel fabs are, I imagine, expensive, which means that sure, manufacturers lead with the new latest-and-greatest, but they have to shift their previous investment at scale and keep shifting it.
Anyway, 4k generally looks better, SD and HD will be upscaled and the panel will probably run at a higher refresh rate. Well, it does on our TV. It took a while for me to adjust, initially I found the picture to look a bit false, like it was computer animation. But once you settle in, it's fine, it's pin-sharp and very detailed. It does make the movie-makers work hard though, it puts actual CGI through its paces. The main thing though, with OLED, is the colour space, it's enormous, from terrifying blacks through to joyfully drenched colours.