The PS4 standalone disc will cost the standard $60, but the Xbox One edition carries an $80 price tag. The price difference comes down to the Xbox One edition requiring a "Legacy Game Controller Adapter." This adapter allows players use their existing Xbox 360 guitars and drum sets; it supports up to four controllers simultaneously, and also works with a variety of third-party instruments.
For PS4, no adapter is required. Legacy Rock Band and third-party guitars and drum sets will work just fine out of the box without any additional hardware.
Harmonix has also published a handy Rock Band 4 instrument compatibility chart to help you determine if those guitars in your closet will work with the new game. Note that the chart is not yet finalized.
Preorders for the standalone Rock Band 4 discs will open today at retailers such as Best Buy, GameStop, and Amazon. In addition to standalone discs, you can buy Rock Band 4 bundles that come with the game and a guitar ($130) or another that includes the game, a guitar, and drum set ($250).
By comparison, Activision's Guitar Hero Live will not offer a standalone disc version, as that game is not compatible with legacy hardware. Instead, you can pick up the game and a guitar for $100 or the game and two guitars for $150.