In oxyacids of chlorine as oxygen content increases. The acidic strength increases.
HClO4 > HClO3 > HClO2 > HOCl
Conjugate base
ClO4−−4R.S.
ClO3−−3R.S.
ClO2−−2R.S.
ClO−−noR.S.
(conjugate base) anion stability increase due to resonance. So acidic character increase.