4k on 1080 sequence shows cropped on Source monitor

This is by design when opening 4K from a HD timeline in the Source monitor. A bit of a nuisance.

I often use the Source Monitor for clips already in the timeline.

Alternative, I use the F key as mentioned before.

If you're doing this just to play with in/out, why aren't you using the Trim functions of the app?

If you set the Preferences/Trim dialog option to "Allow selection tool to choose Roll and Ripple trims without modifier key" it gets pretty slick.

Hover over  cut point, you get the double-ended red arrow icon for the Roll trim. "Roll" cuts leave the two clips where they are, it changes both the out point of the first clip and the in point of the next clip. Click and drag either way, and watch the cut points change between the side-by-side image in the Program monitor. Release when you like the cut.

Hover next to the cut point, you get the yellow bracket for the Ripple trim, which adjusts the near-end of the clip you are 'over'. And slides the clips past either direction to keep a closed timeline. And again, will show in the program monitor as a side-by-side of the two clips at the cut as you're holding it at the time. Release, that's your new cut.

And you don't need to go to the Source monitor.

Neil

It looks to me like you clicked on your 4k clip in your timeline and dragged it to your source monitor. Not sure why you are doing that, but when I just tried with some 4k footage I got the same result as you are getting.

Instead, park your Current Time Indicator (CTI) - otherwise, known as "The Playhead" in your timeline over the clip you want to edit in the source monitor and press the "F" key for "Match Frame". You should see your clip open in your source monitor at a proper scale within and out points set.