Having trouble matching 4K video (3840x2160) to an expected 4000x3000 source output

Newish to Adobe premiere pro, and definitely new to export sizes, aspect ratios, video specs, etc. Have been trying to create a video in 4K res for youtube, but when I go to export it shows the dreaded black borders around the video, and the video is shrunk in the middle. I believe the problem is the source res (4000x3000) does not match the output res (3840x2160) and this is causing the black borders around it. I've tried matching these up by using the custom preset 'H.264 - Sequence#', but the black borders still exist. When I go back and check the video, (right click on video clip and go to properties), it shows the resolution is 3840x2160, however when I go to export it shows it in he source res as 4x3... super frusterating, have been trying to get this to work for several hours now... anyone have any solutions? Footage was shot with a GoPro Hero 10. Also, I just created a video right before this one with other footage shot on the same day using the same camera in the same resolution and edited it all with adobe premiere pro and exported it as 4K using the preset 'High Quality 2160p 4K' and it came out perfectly/normally. Not sure what's going wrong on this one.

Lastly, I tried going to 'effects controls', unchecking the 'uniform scale' box, highlighting 'motion', then clicking and dragging the video corners back to match the total space in the video, but this just stretches out the video and makes it look wonky... has to be a simpler solution that I'm missing here...

Thanks!!