The netflix addon now supports 4K?

No, it’s not fully supported in Windows nor in any unsupported TV box. Windows and unsupported boxes have WideVine L3. To have full Netflix support the device needs WideVine L1 DRM protection and Netflix certification.
Only nVidia Shield, FireTV, Mibox and most flagship phones running Android have all that.

Everything comes down to money. To get NF certificate manufacturers have to pay more money for license than NF makes from it’s TV box users.

You are a dreamer.
NF wants a lot of money for certification.
Do You want to pay it ?

Many have already contacted Netflix over the years and they are not interested because they are making sufficient profits not to want to consider the option.

They would likely argue, in addition to the whole anti-piracy thing that they predominately use as an argument, that TV boxes that are not from well known global vendors may be unreliable, dangerous and other such crap.

It is entirely possible that they may have financial ties to companies that produce hardware and would want to protect other financial interests too.

Bottom line is that you will be wasting your time trying right now.

I have an s905x3 and an s912 with coreelec and the netflix addon. The max I can get to work smoothly is 720p on both. Is there a trick to getting it to do 1080p? If I set it to 1080p, I get lots of dropped frames.

It’s been awhile since I tried, but I’m pretty sure the cpu wasn’t close to being maxed out, especially on the s912. I wonder if something else is the bottleneck.

No trick, Yor CPU is not strong enough
1080p software decoding needs S922X CPU