In a recent YouTube video from blakescars2 you can briefly see the part number on the rear 4 inch speakers isRegarding the rear c-pillar speaker size:
MackrickEcoBoost posted the part numbers for the six speaker and B&O systems. The rears for both are the same part number: M2DZ 18808-J (2) Rear Speakers
Ford parts site indicate that this speaker is used in the front and rear doors of the 2020 Lincoln MKZ.
Crutchfield's "What fits my vehicle" utility for the 2020 Lincoln MKZ indicates that these are 5-1/4"
Maybe the rear speakers are upgraded to 5.25 inch for the b&o?