Ps4 – buy Rock Band 3 now and get the songs into Rock Band 4

Currently:

What we know:

• Most, if not all, of the legacy catalogue of RB(meaning all the DLC along with the past games) has been charted with the cymbals one uses with the pro drums. If you look at the animations of the drummer in-game, you'll notice that the drummer will play on the cymbals or on the toms on notes of the same color. This is not conjecture based off of that, however; HMX reps confirmed this.
• Everything new with the keyboard and guitar controllers will only be available for RB3 and all forthcoming DLC(from RB3). Same source.
• "There are a lot of cues and “hooks” in [HMX's] authoring flow that have always been there; [they] just haven’t had the software cycles to take advantage of them". Again, same source.
• Harmonix will be re-releasing some old DLC with full support for RB3's new features. Currently they are asking for the community to chime in on which songs they would like to see re-released first.

What we don't know:

• What exactly the cues and "hooks" are aside from the drums. I have, however, noticed in-game that there are times in RB2 when the guitarist and the lead singer will sing together, implying that some of the songs in their legacy catalogue may have support for vocal harmonies in RB3. No concrete evidence, though.

Updated August 16th: HMX will be re-releasing old DLC with full support.

It seems Minamimoto's is the best solution. Last year I bought Rock Band 2 Special Edition (bundle with one guitar, one mic and one set of drums) for $99 on GameStop, but they don't seem to be making that deal at the moment (at least not online). On Amazon, it's $113. The Beatles instruments are prettier, so I think this set is worth the extra buck.

As for the other instruments, consider getting Lips. At $30, they come with 2 wireless microphones.

Harmonix is only selling one Rock Band 3 bundle, with the keyboard controller, so I guess you don't have much of a choice there. And it's sold out at Amazon, which is even worse.

As for the cymbals, both RB2 cymbals and the new Pro Cymbals are being sold by the same price, $40. The only issue is that the new Cymbals will only be released on October 30th. According to a review, the new cymbals are much superior, so it might be worth the wait.

The only thing missing is the guitar. There are several options. You could pick a RB2 Wireless Guitar for $60, not much to go wrong there. With $20 more, you could get the Beatles Guitar, which could be nice if you get the rest of the set. If you prefer the Guitar Hero controller, it would be the cheapest option, at $50. And, for $70, you can get the Guitar Hero: Warriors of Rock guitar bundle.

You'll be spending at least $360 on all this stuff. But your parties are never gonna be the same. :-)

EDIT: Ah, and if you want to crank it up to eleven, get this setup.