Harmonix say the ability to transfer songs from Rock Band 3 to 4 is being worked on now and is expected soon. The next update is on December 8th so let's hope it's included.
"Any tracks from the Rock Band 3 disc with authored guitar solos will feature upgraded Freestyle Guitar Solo and Freestyle Vocal features. For those of you who haven't played Freebird with Freestyle Guitar Solos (which is everyone except for the Harmonix staff), you are in for a real treat."
Verified purchasers of Rock Band 3 only will be allowed to transfer songs using the feature which is priced at $14.99.
Rock Band 3 be removed from sale on October 30th on the Xbox Live marketplace and in store until stock runs out.