When editing with 4k footage, make sure your sequence is in 4k, and for any 1080p footage, right click and press “Set to Frame Size” to set it to the correct resolution. Any graphics should also be exported in 4k.
Then when exporting, make a new sequence with the desired output resolution and export your final video from there.
The reason is that when working with 4k footage in a 1080p sequence, and you use an effect on an adjustment layer like “Transform” and then scale up, Premiere treats that footage as if it is 1080p. The result of that is that the scaled up footage now has a combined resolution of less than 1080p, and looks blocky, blurry, and low-res.