  We investigate two types of nonlinear interaction between collinear
femtosecond laser pulses with power slightly above the critical for
self-focusing $P_{cr}$. In the first case we study energy exchange between
filaments. The model describes this process through degenerate four-photon
parametric mixing (FPPM) scheme and requests initial phase difference between
the waves. When there are no initial phase difference between the pulses, the
FPPM process does not work. In this case it is obtained the second type of
interaction as merging between two, three or four filaments in a single
filament with higher power. It is found that in the second case the interflow
between the filaments has potential of interaction due to cross-phase
modulation (CPM).
