The fourth (and last) set of four 4x4 problems...!
Please find Blacks best result!
(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[aa][ab][cb][ac][cc][cd][dd]AB[da][bb][db][bc][dc][ad]C[Black to play - what's the best he can do?]
(;B[ca]CR[ca]
;W[ba]CR[ba]
;B[ca]CR[ca]
;W[bd]CR[bd]
(;B[ac]CR[ac]
;W[db]CR[db]
;B[ba]CR[ba]
;W[ab]CR[ab]
;B[aa]CR[aa]C[Seki! Black wins by one point - 5 prisoners against 4. RIGHT])
(;B[db]CR[db]
;W[ac]CR[ac]
;B[ab]CR[ab]
;W[ba]CR[ba]
;B[ad]CR[ad]
;W[da]CR[da]C[White wins!]))
(;B[ba]CR[ba]
;W[ca]CR[ca]
(;B[db]CR[db]
(;W[bd]CR[bd]C[Black loses the capturing race])
(;W[ab]CR[ab]C[This is a mistake!]
;B[dc]CR[dc]
;W[da]CR[da]
;B[db]CR[db]
;W[aa]CR[aa]
;B[ac]CR[ac]
;W[ab]CR[ab]C[Jigo!]))
(;B[bd]CR[bd]
;W[ab]CR[ab]C[Black loses the capturing race])))