DO NOT BUY Dasung. I spent around $1000 on their Paperlike, and it crashes after about 20-30 seconds the first time, and after 2-3 frames subsequently. I tried troubleshooting this with them, and they had me try different things, updated firmware, etc., until the 120-day deadline with PayPal passed (60+ days of this was purely shipping) -- and then, they're just ignoring my e-mails.
SONY DPT RP1 has a very limited functionality. Only read existing PDF files, and write things on the screen. No additional apps, no functionality as a monitor, anything. However, if all you need it for is to read PDF's, then it is very fast and stable -- highly recommend.
Good e-reader 13.3" has Android, which means that you can run other apps also. At the same time, it sorta implies that it will be slower at best, if not buggy.
Onyx Boox Max Carta has Android, and the additional option to create your own software for it, making it the most versatile of the four.
I haven't tried out Icarus A4, since it's still in the production stage. There is still a possibility that it might turn out to have a story similar to Dasung.