Since the dimensions are being mentioned...
Originally Escape used a 1.5" wide by 3" tall steel box-section frame - this is being referred to as a 3" frame. The rear section is stepped up 3" by overlapping the rear section of each frame rail on top of the front section, and welding them.
Starting a couple of years ago (described in the recall information), all but the rear section was changed to 2" wide by 4" tall on all models (the 21' and 5.0TA already had this size from the beginning of those models) - this is what people mean by a 4" frame. The rear section remains 3" tall in all models.
I don't think the wall thickness has been published by Escape, but as I recall someone reported that it is 3/16 of an inch.