  Fermionic valence bond approach in terms of SU(4) representation is proposed
to describe the $J_{1}-J_{2}$ frustrated Heisenberg antiferromagnetic (AF)
model on a {\it bipartite} square lattice. A uniform mean field solution
without breaking the translational and rotational symmetries describes a
valence bond spin liquid state, interpolating the two different AF ordered
states in the large $J_{1}$ and large $J_{2}$ limits, respectively. This novel
spin liquid state is gapless with the vanishing density of states at the Fermi
nodal points. Moreover, a sharp resonance peak in the dynamic structure factor
is predicted for momenta ${\bf q}=(0,0)$ and $(\pi ,\pi)$ in the strongly
frustrated limit $J_{2}/J_{1}\sim 1/2$, which can be checked by neutron
scattering experiment.
