In which case pH will not change on dilution

Mixture of sodium acetate and acetic acid is a buffer of pH value equal to ${\mathrm{pK}}_{\mathrm{a}}$ so its buffer capacity is very high and hence its pH will not change significantly while ${\mathrm{CH}}_3{\mathrm{COONH}}_4$ is a salt of weak acid ${\mathrm{CH}}_3\mathrm{COOH}$ and weak base ${\mathrm{NH}}_4\mathrm{OH}$ whose magnitude of ${\mathrm{K}}_{\mathrm{a}} \mathrm{and} {\mathrm{K}}_{\mathrm{b}}$ are equal. So its pH does not depend upon concentration. Furthermore, ${\mathrm{NaH}}_4{\mathrm{PO}}_4$ is, in fact, a single solute buffer.