Ok, let me see if I understand you correctly.
I had set the video mode to 640x480 (4:3), which is a Video Mode preset in Shotcut.
I put a red color clip on V1 to show the difference in aspect ratios.
On V2, I have a 4-second video clip with a resolution of 720x576 (5:4)
I went to properties as you have done and changed the Aspect Ratio to 4:3. This will distort the original video.
Another way to accomplish the same is with filters. Filter the track (V1 or V2, etc). In this example V2 is the track I have filtered. Use the Size, Position & Rotate (English) filter. Sorry, I don’t know the German translation.
No clips on this track have a filter applied to them.
Without the filter for the green clip with the 4:3 aspect ratio Video Mode: