
OK, so you are in a 'low magnification' range (4x to be exact), so paper/easel/baseboard problems could possibly be evident. We can still use that equation to find the smallest aperture to bring everything in focus (just like focusing a view camera). The 'dv' (which I neglected to mention is in millimeters) would be the amount of 'crowning' evident in your easel (assuming, as you said, your negative is perfectly flat and everything is in appropriate alignment).
So, if you have about 4mm of crowning the equation would work out like this:
dv = 4mm
M = 4 (16x20 print from 4x5 negative)
So the minimum aperture needed (taking into account both diffraction bullseyes and out of focus circles of confusion) would be:
N_max = (20/5) * sqrt(4)
N_max = 4 * 2
N_max = f8
Likewise, if you can't get it all in focus at say f22, your easel must be crowned up 30 millimeters from the corners.
Can you confirm that when the easel is removed everything is correctly focused on the baseboard? Something other than your easel seems amiss (based on the info at hand).