Please find the best result for White!
(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[bb][cb][db]AB[ca][bc][cc][dc][bd]C[White to play - what's the best he can do?]PL[W]
(;W[ab]CR[ab]
(;B[ac]CR[ac]
;W[ba]CR[ba]C[White wins by one point RIGHT])
(;B[ba]CR[ba]
;W[ac]CR[ac]
;B[aa]CR[aa]
(;W[cd]CR[cd]
;B[dd]CR[dd]
;W[da]CR[da]
;B[ba]CR[ba]
;W[ad]CR[ad]C[White wins big.])
(;W[da]CR[da]
;B[ba]CR[ba]
(;W[cd]CR[cd]
;B[aa]CR[aa]
;W[ca]CR[ca]
;B[ba]CR[ba]C[This is a 5 point win for White, but with this first black move (b4), he can do even better.])
(;W[dd]CR[dd]
;B[ca]CR[ca]
;W[aa]CR[aa]
;B[ba]CR[ba]C[This is a 5 point win for White but after this first move by Black (b4), White can do even better.]))))
(;W[ac]CR[ac]
;B[ab]CR[ab]
(;W[ba]CR[ba]
;B[ad]CR[ad]
(;W[da]CR[da]
;B[ac]CR[ac]
;W[cd]CR[cd]
;B[dd]CR[dd]C[Seki, Black wins by one point (two prisoners against one).])
(;W[cd]CR[cd]C[Seki, Black wins by one point (two prisoners against one).]))
(;W[aa]CR[aa]
;B[ba]CR[ba]
(;W[ab]CR[ab]
;B[ad]CR[ad]C[Black wins!])
(;W[da]CR[da]
;B[ba]CR[ba]
(;W[ca]CR[ca]
;B[ad]CR[ad]C[Black wins!])
(;W[ab]CR[ab]
;B[ad]CR[ad]C[Black wins!])
(;W[cd]CR[cd]
;B[ab]CR[ab]C[Black wins!])
(;W[dd]CR[dd]
;B[ab]CR[ab]C[Black wins!])))))