  We calculate the effective tachyonic potential in closed string field theory
up to the quartic term in the tree approximation. This involves an elementary
four-tachyon vertex and a sum over the infinite number of Feynman graphs with
an intermediate massive state. We show that both the elementary term and the
sum can be evaluated as integrals of some measure over different regions in the
moduli space of four-punctured spheres. We show that both elementary and
effective coupling give negative contributions to the quartic term in the
tachyon potential. Numerical calculations show that the fourth order term is
big enough to destroy a local minimum which exists in the third order
approximation.
