Licensing agreements are a tricky thing. Releasing licensed songs via some medium typically entails setting term limits for how long those songs can be sold. This is an issue that Harmonix first came across three years ago with content for their games, and it's still an issue today. On May 5, 2016, 32 songs from The Beatles: Rock Band will be removed from the store and will not be available for purchase again.
If you already own the songs, or if you purchase them prior to the 5th, then you'll be fine. You'll still be able to play and access those songs in the game as if nothing changed. Harmonix hasn't stated anything about these songs coming over into Rock Band 4 and it seems even more unlikely now with the removal of these songs.
Here's the full list of songs that will be removed on May 5:
The Beatles - "All You Need Is Love" The Beatles - "Because" The Beatles - "Her Majesty" The Beatles - "The En" The Beatles - "Maxwell’s Silver Hammer" The Beatles - "Oh! Darling" The Beatles - "You Never Give Me Your Money" The Beatles - "Golden Slumbers" The Beatles - "Carry That Weight" The Beatles - "Polythene Pam" The Beatles - "She Came In Through The Bathroom Window" The Beatles - "Sun King" The Beatles - "Mean Mr. Mustard" The Beatles - "A Day In the Life" The Beatles - "Being For The Benefit of Mr. Kite!" The Beatles - "Fixing A Hole" The Beatles - "Lovely Rita" The Beatles - "Sgt. Pepper's Lonely Hearts Club Band (Reprise)" The Beatles - "She's Leaving Home" The Beatles - "When I’m Sixty Four" The Beatles - "Within You Without You" The Beatles - "Girl" The Beatles - "In My Life" The Beatles - "Michelle" The Beatles - "Norwegian Wood (This Bird Has Flown)" The Beatles - "Nowhere Man" The Beatles - "Run For Your Life" The Beatles - "The Word" The Beatles - "Think For Yourself" The Beatles - "Wait" The Beatles - "What Goes On" The Beatles - "You Won’t See Me"