Haha I'm the one who did the translations for the Pro Shop Gold's Factory wedges.... I guess I wasn't very clear!
The higher the stage the more focused custom work that is done by Sasaya-san... both 4 and 5 are finished with a copper foundation and finished in a special Gold's Factory Black Chrome. Stage 4's raw head is from Himeji then totally handmade right to the finish. According to Gold's the Stage 5 is 100% in house INCLUDING the forging. They do not reveal much about this but I'm guessing stage 5 would have a different feel from all the other stages with Gold's own forging (hence the high cost). I don't know if it's me but from the pics, the texture of the steel on 5 looks different from 4 even though they are the same foundation and finish.
idrive if you don't hit the 5, I guess we'll never know for sure!! (^_^)