Hi,
I've been using FG knot for about 10 years (I didn't know its name at that time), mostly for freshwater lure fishing at the beginning, but I've never tied it on such a light braid (4lb). I've been using braid from PE 1 (12lbs) to PE 12 (170lbs).
regarding the lighter braids that I use from PE 1 to 3 (10-30lb) connected to fluorocarbon leaders, it really is a perfect knot in terms of resistance and castability through guides, should it be a spinning or casting outfit. Most of other knots just don't offer its huge resistance and its amazing wide range of use.
Alexandre