I edited my 4k footage in a 1080p Premiere Pro project.
Now If I render the project in 4k, will the 4k clips keep their Quality or will they downscale to 1080?
There is two option at sequence setting. Video and video previews. If the video set 4k, and the video previews set 1080, i shoud not be downscale at rendering.
To expand on @Dul32's answer:
Below is the settings dialog for a new sequence. The settings for the preview (what you see when you're editing) are at the bottom, the actual pixels settings for the sequence (what you get at the end by default) are at the top.
Note that the size of the sequence also determines the placement of the video clips and any graphic elements, so if you're doing any movement etc., it might be tricky resizing later, so it's worth setting it up correctly at the start.
The preview format is the codec that gets rendered as a playback preview. All I frame MPEG is fast and small, unless you're doing heavy colour owrk and need a 10 bit preview it's probably the best choice.
Remeber that you have to also set the output settings when exporting. If your sequence is set up right you can match the dimensions and then pick your codec.