You can import your 4K segment into the project and then drag it to the timeline screen (or drag it to the new-segment icon at the bottom). Premiere will create a new segment with the correct size and attributes for editing. In the playback window, you can set the temporary playback quality at 1/2 or 1/4 if you're getting playback stuttering. But that won't affect anything else.
As far as exporting with Adobe Media Encoder, I'm not sure. The H.264 options include exporting at the same size and bit rate. So that should cover it. But for most online uses, like YouTube or Vimeo, you'd probably want to use their 720p or 1080p presets.
My main reason for using the P3P with 4K is to grab better-quality still shots (though I use its regular 12MB stills, mostly, for that). And more importantly, to be able to crop and pan videos with Premiere, and still have full HD quality.