Edit your project as usual with your 4K footage.
Go to the Deliver page.
Under Format, ensure the video codec and other settings are chosen for your desired output format.
In the Scaling section, choose Scale Entire Image to Fit. This will downscale your 4K footage to fit within the 1080p frame.
You can also explore other scaling options like Center Crop or adjust custom scaling factors here.
Click Add Render Queue and choose your desired export settings (filename, location) and initiate the rendering process.