I'm new to all of this but I use a 5" and 3.5" plate. With my PC the 3.5" plate with 4" pads is a good combination for me when it comes to spot cleaning. If I'm working a larger area I use the 5" plate with 5.5" pads.
Guess I really didn't anwer your question but I have not seen 4" plates at any of the sites I visit.
3.5" Dual Action Flexible Backing Plate- porter cable 7424XP hook and loop backing plate 3.5 inch.
The Flexible Backing Plate
LC-43-085DA measures 3.5 Inches in Diameter and is perfect for our 4" Spot Buff Pads.
The backing plate is smaller than the pads to allow a generous lip of foam to protect the paint surface. Even if you turn the pad onto its side accidentally, there is an inch of soft, beveled urethane between the edge of the plate and the inner construction. The backing plate fits any polisher that accepts a 5/16” thread.