PMP isn’t being supported anymore, have you tried the newer clients?
HTPC: Introducing Plex HTPC! or Plex for Windows from the download page.
Are you going from your Dell → HDMI → AVR → 4K tv.
There is a technical requirement that the PC’s HDMI out be at least 2.0b to handle the bandwidth required to send 4k video with Lossless audio which might require a newer video card.
