  We consider the sl(2) current algebra at level k=-4 when the sl(2) BRST
operator is nilpotent. We formulate a spectral sequence converging to the
cohomology of this BRST operator. At the second term of the spectral sequence,
we observe an N=4 algebra. This algebra is generated in a c=-2 bosonic string
whose BRST operator Q_{string} represents the next term in the spectral
sequence. We realize the cohomology of the irreducible modules as
Q_{string}-primitives of the N=4 singular vectors and point out their relation
to Lian--Zuckerman states of c=-2 matter. The relation between sl(2)_{-4} WZW
model and c=-2 bosonic string is established both at the level of BRST
cohomology and at the level of an explicit operator construction. The relation
of the N=4 algebra to the known symmetries of matter+gravity systems is also
investigated.
