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]
AW[ca][cc][dc]AB[aa][ab][db][ac][bd]C[No komi, no prisoners - what's the best result for Black?]
(;B[cb]CR[cb]
;W[bb]CR[bb]
(;B[bc]CR[bc]
;W[da]CR[da]
(;B[cb]CR[cb]
;W[db]CR[db]
(;B[ba]CR[ba]C[Black wins! RIGHT])
(;B[cd]CR[cd]
;W[ba]CR[ba]C[Seki]))
(;B[ba]CR[ba]
;W[cb]CR[cb]
;B[cd]CR[cd]C[Seki, White wins by two points (the two prisoners)]))
(;B[ba]CR[ba]
;W[bc]CR[bc]C[White wins]))
(;B[bc]CR[bc]
;W[cb]CR[cb]
;B[cd]CR[cd]C[Seki, White wins by one point (the prisoner)])
(;B[bb]CR[bb]
;W[cb]CR[cb]C[White wins])
(;B[cd]CR[cd]
;W[bc]CR[bc]C[White wins]))