my 2 cents…
it seems to me you’re kind of doubling up on proxies/previews here though that may be necessary depending on your system spec. you could have edited from the native footage in a 1080 sequence, but then you need a fast drive to handle the data rate of the 4K, etc etc.
one issue though is that if you’ve edited from files that are 1080 and relink to 4K then you may have to do some resizing and redo effects/nesting depending on how you’ve worked within your sequence.
i guess among other things, shooting at 4k gives you the ability to reframe and zoom in on footage to a high degree, and also future proofs you if you need to deliver 4K down the line. the 1080 delivery is fine for now but that’s not to say you may not do another version later.
if you can you should grade from the native – they have a lot more colour information in them than the proxies and what you see when grading will be exactly what you get in the end.