Text copied from "Japanese Infantry Weapons" cincpac cincpoa Bulletin 55-45, 15 March 1945 :

"A new, non-metallic hand grenade has been encountered. The cover is light brown pottery like that used in the Type3 Land Mine. The grenade weighs one pound, is three inches in diameter, and is entirely round except for the fuze protrusion at the top.
A waterproof rubber bag encases the grenade proper. A similar sack covers the scratch block at the top of the fuze and is snapped down over the lips of the fuze.
To fire the grenade, remove the rubber sack from the top and strike the scratch block against the protruding match composition".

On the right you see a page from another manual, thanks to Craig.

**Some sources describe this grenade with the name Type4. I don't know which name is correct.
Maybe someone can explain this?
Let me know.