The E4 pieces can be added to the wing after the wing is complete but, before it is removed from the building board.
Then carved to proper shape after removing from the board.
Look on the plans, there is a side view of E4, (3 pieces required). The picture is next to the left wing drawing, near where they show gluing the two piece plastic pilot. The bottom edge of the E4's are carved upward. Picture this: E4 is part of the flap. When the flap is down, E4 must fit the contour of the rear window. Top surface of E4 is the same profile as the rib it's glued to.
Clear as mud??
Not too sure about H5. I think what I did, was glue H5 to the stringer and B4. Then sand it to make a smooth contour.
And then just trimmed and adjust the rear window to fit on top of H5. It's not on the plans but, I added a paper strip, approx. 1/8" wide, all the way around the base of the front and rear windows. It's called a retaining strip on the full size aircraft. This makes a nice transition from the window to fuselage and hides any gaps on the model.
If I get a chance, I will try to get photos of the full size Cessna showing what E4 is supposed to represent.