Assuming the 4k and 1080p discs have the exact same sync (they certainly should), you can always edit the 1080p version and hang on to your project file. Premiere makes it very easy to simply replace a source. So, someday you can come back and upgrade it, tweak the project settings, upscale a few deleted scenes and render.

That will also save you from needing to scrub through the 4k version in the editing process.

Of course, those of you guys doing extensive work in after effects will have a bit more manual work to do. Might be worth starting those kinds of shots at as high a resolution as possible if 4k would ever happen down the road.