8 KOH (aq) + 4 Cl2 (g) → KClO4 (s) + 7 KCl (aq) + 4 H2O (l)
This is an oxidation-reduction (redox) reaction:
7 Cl0 + 7 e- → 7 Cl-I (reduction)
Cl0 - 7 e- → ClVII (oxidation)
Cl2 is both an oxidizing and a reducing agent (disproportionation (dismutation)).