  YbInCu$_4$ undergoes a first order structural phase transition near $T_v$=40
K associated with an abrupt change of Yb valence state. We perform ultrafast
pump-probe measurement on YbInCu$_4$ and find that the expected heavy fermion
properties arising from the \emph{c-f} hybridization exist only in a limited
temperature range above $T_v$. Below $T_v$, the compound behaves like a normal
metal though a prominent hybridization energy gap is still present in infrared
measurement. We elaborate that those seemingly controversial phenomena could be
well explained by assuming that the Fermi level suddenly shifts up and becomes
far away from the flat \emph{f}-electron band as well as the indirect
hybridization energy gap in the mixed valence state below $T_v$.
