Based on a few sources, there is a way to bring customs to RB4. (Note: PS4)
You will need....
A jailbroken PS4 that runs under version 5.07,
Rock Band 4 Version 2.13
a formatted portable hard drive
and Libforge (https://ci.appveyor....aster/artifacts)
Step One: Extract and open Libforge.
Step 2: Go to [Tools] and click CON to PKG
Step 3: Pick the CON file you are converting.
Step 4: There will be a dropdown and a option for [Build for SCEE] (Note: click it if you have the European version)
Step 5: Click [Build] and select the output location
Step 6: Drag the .pkg file in the root of the portable hard drive
Step 7: Go to settings on PS4 with the portable hard drive and press [debug settings] and go to the package installer
Step 8: Find the .pkg file and install it
Step 9: Open Rock Band 4
Step 10: Go to song library and it will show up there.
(This is not made for purposes regarding official dlc)