Since "Gaiden" means "side story" in Japanese, this is likely the fourth volume of a spin-off series focused on a "Spirit Witch" character. The "mxwz" tag usually indicates a specific translation group, scanner, or encoder (common in manga/anime piracy circles). Magdalene St Michaels The Stepmother Vol 5 Her New Son Trailer Target Work Official
"Spirit Witch's Gaiden v04 mxwz" appears to be a specific file name or release title, likely referring to a . Artofzoocom Full Today
It sounds like you enjoyed it! What specifically made it a "good piece" for you?