– the RB's error sound plays a sharp (although not necessarily loud) mishit sound for the note(s) you missed, so you know that the note you hit was wrong. It still plays the louder drum parts you hit correctly on top as well, so you know which part you got right
– In GH4, the error sound overrides the drum sounds in volume so you may not be immediately sure which note you mishit, plus the soundbyte seems to be that of a guitar note being missed, which sounds weird when playing the drums. In addition, the timing of the error sound seems a bit off, but I think that's cause the error sound actually sustains a bit.

– When an instrument track is failing in RB, the entire highway flashes red, making it obvious to the player. In GH4, you have to watch some easy-to-ignore thing on the side
– In RB, when an instrument fails, it can be saved by a teammate's star power (I refuse to call it “Overdrive”). In GH4, Star Power needs to be applied *before* a teammate's failure for it to save the player. This difference seems to make RB more of a fun party game, and makes GH4 seem like it's still stuck in the old paradigm.
– GH4's instruments do seem to have more features: the drum's kick pedal is less easy to break (I snapped RB's one) and it's got an extra pad, and repositioned closer to where you'd expect the hi-hat and crash cymabl to be. However, RB's drums definitely feel more responsive. I haven't played the guitar much on either game, but the extra slider option in GH4 looks cool, but I'm told it's not really used much.
– RB's sound mixing is much better. The GH4 error sounds are more annoying than informative, and the tracks are quite quiet compared to all the other sounds (menu's, or errors). In RB, the tracks are just as loud, if not louder, as the menu which makes it easier to hear.
– The drum samples in RB's drum fill ins are cooler and more rocky than the quiet inoffensive ones in GH4.

A things in favour of GH4:

- Bigger song selection, especially since my flatmate's PS2 doesn't offer downloadable content
- RB, although more fun overall, also provides less challenges. GH4 has a wider variety of difficulty which is good for players who are bored with the levels of difficulty (my flatmate can finish the expert songs without focusing too much in RB, whereas there are definitely songs in GH3 and GH4 that he could not finish).

There's this known issue in the PS2 version of Rock Band, where sometimes if you keep using the instruments while it's trying to load a song, it stays stuck in pre-song mode, in which the drum's are in free-style mode. I actually like this bug, cause it means I can play around with freestyling on the drums for as long as I want.