Jeep - never heard of G4, but suspect is what is called 'Dope' here (w/acetone thinner) - this is used to seal the paper skin on aeroplane wings etc. I used it years ago, and is O.K as a sealer, but does not have the penetration or toughness of Prop or D2T.
I have forund Prop to be very forgiving as a sealer, (tried them all) very hard, easy to sand, and goes a long way - personally I use it disolved in virgin lacquer thinners, which takes longer to dry, but I think, it soaks in, and levels out a bit better - As a sealer, it does not matter much what thinner (acetone or lacquer thinners), as long as you give it time to dry (12-24 hr or more) you will have few problems. 'Swede' is the GURU on this stuff, so maybe if you want some, PM him. I used to use, dissolved 'eye glass' lenses, but this stuff is clearer and better. I am not selling it, just trying to save you the years, I and a lot of other guys have bumbled around with "secret recipes". pete