An undub patch replaces the English voice acting with the original Japanese voice tracks while keeping the English text/subtitles. For RF4 , the 3DS English release had dual audio? No — it was English voices only. So undub patches exist that swap in the Japanese voice files from the Japanese version.
Are you planning to play this on an like Citra, or on original 3DS hardware ? Rune Factory 4 (Undub) - hShop rune factory 4 decrypted 3ds undub rom reg link
The demand for decrypted 3DS ROMs, including Rune Factory 4, has increased significantly over the years. This is largely due to the convenience and affordability of digital game copies. Players can easily download and play the game without the need for a physical cartridge. An undub patch replaces the English voice acting