The HCl is a strong acid, that is, it is fully consumed in the reaction.
HCl(aq) ------------> H₃O⁺(aq) + Cl⁻(aq)
↓ ↓ ↓
10⁻⁴ M 0 0
-10⁻⁴ +10⁻⁴ +10⁻⁴
_____________________________________
0 + 10⁻⁴ M + 10⁻⁴ M
PH = - log [ H₃O⁺ ]
PH = - log [ 10⁻⁴]
PH = 4.0
