The G4, as far as I know is the exact same thing as the G5, except for the "dual band" capability of the G5, both have the same programming capabilities.
What exactly do you mean by channel announcement? A pre-recorded tone or voice that plays when a talkgroup is activated?
If so, I can confirm you can do that on both G5 and G4. It is however an annoying feature if that talkgroup is frequently used specially with quick transmissions between users.
If thats not it, then perhaps the knob voice announcement? If so yes G4 can also do it. Its helpful for those changing knob position without needing to look at the pager.