FYI, I bought a Vizio E series (E55U-D2) and so far I'm very happy with it. It has a pseudo-HDR (called Local Dimming) that I decided I didn't need, but it does work. Attach your 4K source to HDMI1 (I use a SHIELD but a Roku 4 will work fine too) and stream away. The remote is kinda cheap, you'd rather use the Android/iOS app (the M series includes a tablet). The E series has a built-in Chromecast but AFAIK it only works at 1080p, so you need a 4k streamer box. Given how cheap the E series is, that shouldn't be a problem. And I don't like built-in proprietary stuff anyway.
I saw an M series and an E series Vizio side-by-side in the store. While the M series looked a little better, and had a few more features and ports, I decided it wasn't worth the extra money for the same size device. But I was looking at 55 inch, not 43 inch.
Bottom line, I think the M series adds HDR, a 6 inch tablet remote, and an extra HDMI port. At the 55 inch level it was a $300 difference when I was shopping...definitely NOT worth it when I've already got phones and tablets coming out of my ears. and only 3 HDMI devices to begin with.