Dave I miss read the first part of your post. The inside dimension of the Vertical "legs" of the Angle is 4'-1/4" but the "flange leg" turns inward by 1.0625", so the ply wood will actually rest on the inside frame. The inside clearance Angle to Angle is actually 4'-1/4" but you have 1.0625" on each Angle turned inward for the plywood to rest on, in other words the "hole" "inside opening is 3'-10-1/8".
Sorry about that, I read what you posted but my brain was asleep 
