  We evaluate various disk level four-point functions involving the massless
scalar and tachyon vertex operators in the presence of background B-flux in
superstring theory. By studying these amplitudes in specific limits, we find
couplings of two scalars with two tachyons, and couplings of four tachyons on
the world-volume of non-BPS D-branes of superstring theory. They are fully
consistent with the non-commutative tachyonic Dirac-Born-Infeld effective
action. They also fix the coefficient of $T^4$ term in the expansion of the
tachyon potential around its maximum.
