Have you tried "Fit?"
Otherwise, I think that you need to perhaps tell us more about your Sequence settings and your Assets, as something might be amiss there.
The Program Monitor's size will reflect your sequence settings. If you want a 4:3 program, use only 4:3 media and create only 4:3 sequences.
Brilliant...Thank you so much.. A preset I had...
I appreciate speedy reply and assistance all round.