(;
GM[1]
SZ[19]
AW[dc][dq]
AB[qd][pq]
LB[qd:1][pq:3][dq:4][dc:2]
C[Problem 4. Black to play.  

White has played 4 in the lower left corner. What should Black do now ? ]
;B[od]
LB[nc:B][oc:A]

TE[2]
C[Correct Answer.  

Black should take this opportunity to make a corner enclosure with 1.  

Instead of 1, making a corner enclosure with A or B would also be a good move. ]
(;W[qo]
C[Allowing Black to make a second corner enclosure in the lower right corner would give him a large territorial advantage, so White must make an approach move with 2. ]
;B[qk]
C[However, Black can now take the initiative by playing a pincer with 3. This move is also an extension from his corner enclosure above. <= ]
)
(;W[po]
C[Variation.  

If White makes a high approach move with 2, ... ]
;B[pl]
LB[qo:B][qm:A]
C[Black will play a two-space pincer with 3 or a pincer at A. Black 3 at B is also possible. <= ]
)
)