As I wrote, afaik it is the same surface, but not that thick. I once sent an email to a shop asking if there was any difference between the surface of the No.4 and the G4 and they said no which was confirmed by the manifacturer. I have both here and I cannot tell any difference.
So No.4 = G4 and G4 has the same surface as G4 thick.
I have been using Schoelershammer No. 4 Airbrush paper for the last two years and love the stuff, my only complaint with it other than it being cream and not white is that it warps when I start painting, although it does return to flat when it dries which makes it fine for me, I just recently purchased some Schoelrshammer G4 board thinking I would get the same thing but with rigidity, I was wrong, the only thing I liked about it was that it is whiter than the paper, I didn't like it at all, it was almost impossible to scratch without destroying the surface which I found to be a lot softer than the paper, to make it worse it has a texture which I don't want, since I do a lot with scratching I want my lines to be uniformed and unbroken rather than hopping an skipping all over the surface, so I'm afraid they are definitely not the same, I'll stick with the paper since despite the small disadvantages it gives me what I wan't, after one time using the G4 board I knew enough to know it just doesn't suit my needs.
I would actually much prefer to work on my gessoed MDF, but the preparation is the killer, the No. 4 paper is ready to blast other than masking a little border around it, so I'm happy to pay for it since it does save me a lot of time
I also tried some Yupo for the first time and my conclusion there is, although I like easy, there is such a thing as too easy, not at all impressed with it, for what it cost, it was worth it just to try, but what I have left will be used just for free hand shields and practising new methods, as
@AndreZA said somewhere else, our work wouldn't last long on it, your work could easily be removed from it, I did notice that strong bright colours although removable will dye the surface, so it's not even reusable except maybe as stencil material.