I still use my PS3 for bluerays, so if you want to save some money you could give it a try and see how your TV or receiver upscales to 4k if you go 4k. If the 4k streaming content is noticeable better than your bluerays, you can consider a 4k player, but remember that you need 4k discs as well. The player should upscale older 1080 discs as well but whether it does that better than the receiver depends upon the internal electronics.
I also like Plex for streaming video files and it works well with music also. Besides the Roku, the NVidia Shield is a nice streaming device that does Plex, Youtube, and many other services, and with a bit of effort will run Kodi as well.