The 4th analytical group of cations includes ions that precipitate as sulfides at pH 9. The reagent used is ammonium sulfide or NaS 0.1 M added to the ammonia/ammonium chloride solution used to detect group 3 cations. 
It includes: Zn, Ni, Co, and Mn. Zinc will form a white precipitate, nickel and cobalt a black precipitate and manganese a brick/flesh colored precipitate. Dimethylglyoxime can be used to confirm nickel presence, while ammonium thiocyanate in ether will turn blue in the presence of cobalt. This group is sometimes denoted as IIIB since groups III and IV are tested for at the same time, with the addition of sulfide being the only difference.