How to export 4K into 1080?

Either workflow will work. Premiere Pro allows you to change the frame rate, resolution and even the aspect ration during export. You will not find more export settings in a NLE than what Premiere Pro has to offer. The video below demonstrates this at about two minutes in.

put the 4k into a full hd sequence and export full hd.

: )

don't upscale... that interpolation is handled better at your TV end than from editing program.

Off topic, but it should be said that some want to do the opposite. Shoot and edit in 1080 but upload to YouTube in 4K. I say, try it and check those results.

Yes, you can shoot and edit in 1080 and blow up the sequence to 4K. I do that in Media Encoder. Some say they cannot see the difference between a scaled up 1080 cut from 1080 source and one that originated in 4K and was exported in 4K. I think that's pretty cool. Yay, Media Encoder!

Kevin

Just set the Preferences option for scaling to "Set to framesize" and use a 1080 sequence. The clips will all "land" with the image filling the sequence borders. And you can re-size without the hit of resolution drops you would get from "Scale to framesize" as a preference.

Neil

Thank you very much!

Either workflow is fine.  You will by it's intrinsic nature lose quality when you downrez from 4k to 1080, but it will at least be very close in the quality you would have gotten  if you shot on 1080 to begin with.