OBBExtract is a tool written by RadiantDerg that can extract the files from the OBB file in the Android port of Sonic the Hedgehog 4: Episode II. This tool cannot repack the OBB file. The source code of OBBExtract (and other tools) can be found here, and a precompiled version of OBBExtract can be downloaded from here.
To use the tool, simply drag and drop the game's OBB file onto OBBExtract's executable file.
The Sonic 4 Bonus Site is not associated with SEGA, Sonic Team, or Dimps.