Ability to downscale 4K to 2K

I have 4K content, and would love to downscale the quality to 2K (1440p) so I can play it on my MacBook.

Do you mean (1) on-the-fly or (2) in advance?
Both is already possible.

  1. install PMP on your computer and use this as a client (requires your computer to have a certain set of playback abilities)

  2. use Plex’s optimize feature (just requires time and computing power on the server)

So the optimize only allows max, 1080p. There’s no option with the 4K to optimize it in 1440p…