Welcome, Jared!
Yes! You can do exactly that.
Simply deselect the « Always transcode original resolution » box and select the 1080p box on the page « admin/config/edit-custom#transcoding ». Now if someone uploads a 4k video, it will be automatically downscaled to 1080p and PeerTube will discard the 4k video file (unless you enable the save original video file feature), saving you storage space.
If you were to do the same but with the original resolution box selected, it would transcode both a 4k video and a 1080p version.