I have both RB2 and GH4 for the PS3 and I enjoy RB2 a lot more for various reasons. Also, GH4 drums don't work on RB2 for the PS3. But anyways, GH4 has a lot of potential for greatness, not the least which being its slightly superior set list, mostly due to Crazy Train, Beat It and Hotel California. Dam I love Crazy Train. For the most part, my main issue with GH4 is that once again, it is much more cartoony than its Rock Band counterpart. With the previous Guitar Heroes, this wasn't so much an issue. However, now that you can create your own characters, this kid-friendly atmosphere is a lot more noticeable, especially after playing RB2 which I can say with total honesty is superior in both character customization and overall 'look'. I would even go as far as saying that GH4 look is less than that of RB1, but whatever floats your boat.
Like nearly all rhythm games within their first few weeks of release, the instruments have issues, the most prominent in my mind the drum pads. GH4 drums vary wildly, my own set has a nearly unresponsive blue pad, and I have to tape my cymbals down to get a consistent sense from them. The RB2 drums are just overly sensitive, which is actually kinda worse, because there isn't much you can do to go around that. I'm sure these issues will be resolved soon so don't let them factor into your purchase decision too much. Also, the GH4 guitar is sexy as hell, pick it up even if you're getting RB2.
GH4 game play is much like Rock Band 1 and 2, the only truly marked difference is singing, which is kind of unintuitive for first time singers as talking and singing parts are distinguished by a slight color change, and you don't know how well your doing on a particular section until you either receive or don't receive the bonus multiplier.
Basically, all the parts for a great game are present, and the superb song list only adds to this. Which actually makes the game that much more disappointing. Rock Band 2 was basically just an upgrade to the graphics of Rock Band 1 and an increased song list, but at least the game was fun. Guitar Hero 4 is frustratingly disappointing. In band play, once one person fails, the entire band fails, no matter how well everyone else was doing. This is even more frustrating on online play when songs are chosen in turn by each player, so you can easily end up playing a string of boring songs and then fail 30 seconds into your choice. Also, it is nearly fucking impossible to activate drum star power without royally screwing yourself, as you have to hit both cymbals at the same time. The one place Guitar Hero 4 truly shines is the guitar, which is to be expected, considering the many years of refinement they've had. I praise to heaven whoever implemented the star power button.
Overall, Guitar Hero 4 has alot of potential to be great, but i believe it fails in too many areas to justify a purchase, especially considering that Rock Band 2 is within the same price range and is much much better. Guitar Hero 4 should have stuck to just guitars, the addition of the other instruments is an understood attempt at challenging rock band, but fails because it lacks the precision and refinement that Rock Band 2 was been able to achieve. It's too bad you can't give Rock Band 2 the song list from Guitar Hero 4. Maybe Guitar Hero 5 will be better.