OK. Personally, I doubt that streaming- even with HEVC- is going to compete with disc quality. I suspect h.265 offers the opportunity for that idea but I suspect it will be squandered to go for "thinner" instead of filling the "added capacity" with more detail. I suspect h.265 at 4K is simply going to be striving for comparable file sizes as h.264 for 1080p... that someone somewhere in charge of such thinking is targeting some arbitrary bandwidth capacity target, and thus jamming 4K resolution into nearly the same amount of virtual (file) space as 1080p in h.264 containers.
That written: if you believe h.265 is going to bring higher quality streams but you are happy with your Panasonic, there is nothing I know of to stop you from opting for the 4K file stream to then be downconverted to the 1080p Panasonic screen. It will deliver the quality in that stream to the fullest it can be displayed on your Panasonic. Buying a 4K TV is not going to automatically improve the quality of the stream, nor is it necessary to access the (presumably) better quality file.
In short:
- if quality is still king for you, discs is still THE way to go
- if you perceive HEVC streams will deliver better quality, you can opt for those and feed them to a new 4K set or the existing 1080p Plasma. Yes, the latter won't be playing them back at 4K but the downconversion from 4K should yield a very impressive 1080p picture. And if there is more actual data in the 4K stream, it will likely show in a better 1080p picture than streaming the h.264 version of the same file.