The 4D32 manual is often bundled inside the broader "Mitsubishi Fuso Canter Chassis & Engine Shop Manual." If you cannot find a standalone 4D32 PDF, search for the Fuso FE or FG series workshop manual corresponding to the late 1980s to late 1990s models.
While robust, the 4D32 isn't invincible. Mechanics often look for the manual to solve these common issues: Overheating: Mitsubishi 4d32 Engine Manual Pdf