Zor,
F4 is the front piece that makes the windshield. You may be talking about where the fuse starts to taper in running towards the rear? If so, there are doublers on each side. and at the point where the fuse sides begin to taper, the doublers have small cuts to be bent. The directions are a little unclear as what to do. I states to cut into the part at the dashed line, and glue. But what I did was left it unglued till the fuse was done, and then glued them( the small cut off part) in.
When I finish the wing covering, I will take a picture of the doublers to help explain.

Quigleywins, I am covering the wing with solite from mountain models. This film is very light, and took me some time to learn how to work with it. I have found that using low heat to make the glue stick to frame, then turn up heat and shrink. It does have a lower heat rang than other coverings, so you do have to be carefull when using it.