Hey, guys, I'm unsure if this has been mentioned yet (the search function seems iffy), but I received an email from Harmonix about the new bundles for Rock Band 4 being up for preorder now. Here's their official site's information on it, and it looks like there's a link to both Amazon and Gamestop for preordering. The expected release date is October 6, 2015.
Now, before you have a heart attack, know that they're fairly expensive. The Band-in-a-Box Bundle is $249.99 and includes a wireless guitar, wireless drum kit, a USB microphone, and the game, whereas the Guitar Bundle retails for $129.99 and includes the game and a wireless guitar only. Here's a link to the bundles:
From the looks of it, preordering the game gets you 30 additional tracks, with Amazon offering an extra 4 to that total (making it 34). Here's what Amazon says about the extra tracks:
In addition, I've heard that Harmonix plans to release a "low-cost" adapter so that you can use your old wireless instruments with the new systems (Xbox 360 and X1 use different wireless protocols, at least), but I haven't seen any mention of a price or a listing yet. Additionally, most of the past DLC from the previous games will be compatible with this one (and is the main reason I bought an X1), but at least 150 songs will not be compatible.