Tetrabasic acid means acids having 4− replacable hydrogen atoms.
H3PO2⟶ hypophosphorus acid
Metaphosphoric acid⟶(HPO3)n
Pyrophosphoric acid ⟶H4P2O7
Orthophosphoric acid ⟶H3PO4
Here, pyrophosphoric acid, H4P2O7 have 4 removable H+ ions, hence is a tetrabasic.