A quick Google search throws up a couple of articles that support my understanding:

http://www.sportsandsafetysurfaces.co.uk/blog/5-a-side-football-pitch/difference-in-3g-4g-5g-6g-synthetic-artificial-pitches

https://blog.mylocalpitch.com/2g-3g-4g-know-playing-surface/

World Rugby's website says the following:

"What is the difference between 3G and 4G artificial turf systems?

3G (third generation) is a term used to describe the current technology used in creating an artificial turf system comprised of yarn laid on a backing, plus sand and infill. The term 4G can usually be found in marketing materials and is used primarily as a sales tool but is not an accurate representation of any system currently on the market."

The transition from 2G to 3G was about the move from sand to rubber crumb. As far as I'm aware, there has been no such further development to justify a rebrand from 3G to 4G, 5G or 78G. Furthermore, there is nothing other than companies' own marketing spin to say that more expensive means better quality (however you quantify that) or more importantly in the context of the discussion improved safety.