Please find the best result for Black!
(japanese rules)
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[4]KM[0.00]
PW[Weiß]PB[Schwarz]AW[da][bb][ac][bc][dd]AB[aa][cb][db][cc][ad]C[Black to play - what's his best result?]
(;B[cd]CR[cd]
(;W[ba]CR[ba]
(;B[bd]CR[bd]C[White can't play c4, so he is dead RIGHT])
(;B[ca]CR[ca]
;W[bd]CR[bd]C[This is only a jigo....]))
(;W[ca]CR[ca]
;B[ba]CR[ba]
;W[ca]CR[ca]
(;B[da]CR[da]
;W[bd]CR[bd]
;B[dc]CR[dc]C[Seki, Black wins by three points (4 prisoners against 1)])
(;B[bd]CR[bd]
;W[ab]CR[ab]
;B[ba]CR[ba]C[White is dead])))
(;B[ca]CR[ca]
;W[bd]CR[bd]
(;B[ba]CR[ba]
;W[cd]CR[cd]C[This is only a jigo - seki....])
(;B[cd]CR[cd]
;W[ba]CR[ba]C[This is only a jigo]))
(;B[ba]CR[ba]
;W[cd]CR[cd]C[This is only a jigo - seki])
(;B[dc]CR[dc]
;W[ba]CR[ba]
(;B[cd]CR[cd]
;W[ca]CR[ca]C[White wins!])
(;B[ca]CR[ca]
(;W[cd]CR[cd]
;B[bd]CR[bd]
;W[cd]CR[cd]
;B[bd]CR[bd]
;W[ad]CR[ad]C[Seki, White wins by one point (4 prisoners against 3)])
(;W[bd]CR[bd]
;B[cd]CR[cd]C[White wins by one point!])))
(;B[bd]CR[bd]
;W[cd]CR[cd]
;B[bd]CR[bd]
;W[ba]CR[ba]
(;B[ca]CR[ca]
;W[ad]CR[ad]C[Black is lost])
(;B[dc]CR[dc]
(;W[ca]CR[ca]
;B[cd]CR[cd]
;W[ab]CR[ab]C[Seki, White wins by one point (3 prisoners against 2)])
(;W[cd]CR[cd]
;B[ca]CR[ca]
;W[ad]CR[ad]C[Seki, White wins by one point (4 prisoners against 3)]))))