I am keeping track of my research into RB4 customs here.
I am working out the structure of the files in Rock Band 4. These are being documented in the form of 010 Editor Template files, which are in the 010 directory.
Can be downloaded at Appveyor
This is a library I'm working on that handles reading, writing, and converting for formats in the Forge engine used by Rock Band 4 and Rock Band VR.
It is licensed under the GNU LGPLv3 and includes two frontends at the moment:
This is a command line tool that currently does file conversions from rbmid -> mid, rbmid -> rbmid, mid -> rbmid, and {png|bmp}_{ps4|pc} -> png
This has an ark file browser with texture preview, songdta preview, and DTA preview.