Why does the iupac name of [Cu(NH3)4]SO4] only have tetra and not tetrakis?????

Dear student
The IUPAC name of the given compound is Tetraamminecopper(II) sulfate
Here the ligand is Ammonia, which is a simplae ligand. 

The prefixes ‘bis’, ‘tris’, ‘tetrakis’, ‘pentakis’, etc. are  used for bidentate ligands. and with composite ligand names in order to avoid ambiguity, for example if the complex has ethylene diamine, which is a bidentate ligand then the nomenclature would be:

[Pt(H₂NCH₂CH₂NH₂)₂Cl₂]Cl₂

Answer: dichlorobis(ethylenediamine)platinum(IV) chloride

Regards