I have dyed a few Fashion Style G4's and they turned out ok, but you have to leave the head in a few seconds longer. Also, get that dye hot, boiling hot, cook them ponies, otherwise it just doesn't like to stick. I put a folding steamer in the bottom of the pot to prevent burning (learned the hard way). Also, they will fade a little, so dye them slightly darker then you want them, the Berry Punch one shown is right after it dryed, it's now faded to a more show accurate shade.
As for the non FS G4's, I have had mixed results dying those, not sure why. The heads just do not want to take dye. I resort to painting them.
BTW, as someone else said, use powered RIT. In my experience, the liquid stuff just isn't very strong and for the most part useless.
Here's my G4 FS Rarity dyed a brownish-grey
And a G4 FS Pinkie Pie that I'm making a Berry Punch out of (and having to re-hair from scratch after cutting this hair too short while styling it *grumble*) for someone.