Harmonix has announced that Rock Band 4 will start to rock the world this October on Xbox One and PlayStation 4, with 60 tracks included on the retail disc.
You will also have access to an existing library of 1500 tracks which you already purchased and are compatible with Rock Band 4. You can also use your old instruments with the game too if you still have them.
You can already pre-order your copy of Rock Band 4 and if you do this, you will receive 30 bonus songs, Rock Band 4 will launch for Xbox One and PlayStation 4 on October 6th 2015.
Source: Rockband Official Website