hello piotr and welcome, only been here a month myself too.
One thing that I have noticed is how many hours I can lose playing with the search function near the top of the screen. It is very good and you can easily answer quite a few questions by having a look through the results. It is like walking into a great big pub full of window cleaners, ask it a question and it delivers a multitude of views on almost any subject.
As for GG4, not used it. Search says a lot of stuff about it being the "hard water" version where as GG3 is the "soft water" version. Also a lot of stuff about their being a lack of glide but an amazing finish if you are willing to perfect it and a few folks find a tiny bit of fairy gives the glide back.
but thats not me speaking, it is the search function. Its a fantastic tool.