  The nuclear symmetry energy coefficient (including the coefficient $a_{\rm
sym}^{(4)}$ of $I^{4}$ term) of finite nuclei is extracted by using the
differences of available experimental binding energies of isobaric nuclei. It
is found that the extracted symmetry energy coefficient $a^{*}_{\rm sym}(A,I)$
decreases with increasing of isospin asymmetry $I$, which is mainly caused by
Wigner correction, since $e^{*}_{\rm sym}$ is the summation of the traditional
symmetry energy $e_{\rm sym}$ and the Wigner energy $e_{\rm W}$. We obtain the
optimal values $J=30.25\pm0.10$ MeV, $a_{\rm ss}=56.18\pm1.25$ MeV, $a_{\rm
sym}^{(4)}=8.33\pm1.21$ MeV and the Wigner parameter $x=2.38\pm0.12$ through
the polynomial fit to 2240 measured binding energies for nuclei with $20 \leq A
\leq 261$ with an rms deviation of 23.42 keV. We also find that the volume
symmetry coefficient $J\simeq 30$ MeV is insensitive to the value $x$, whereas
the surface symmetry coefficient $a_{\rm ss}$ and the coefficient $a_{\rm
sym}^{(4)}$ are very sensitive to the value of $x$ in the range $1\leq x\leq
4$. The contribution of $a_{\rm sym}^{(4)}$ term increases rapidly with
increasing of isospin asymmetry $I$. For very neutron-rich nuclei, the
contribution of $a_{\rm sym}^{(4)}$ term will play an important role.
