4. A football made of leather in black and white colors is shown as follows. It is known that this football has 12 black leather pieces. Therefore, this football has  white leather pieces.

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4. 20 (Hint: Each black piece is a pentagon, and the total number of edges for 12 pieces is: $5 \times 12=60$ (edges). Each white piece is a hexagon, and three of its edges are adjacent to the edges of black pieces. That is, three edges of black pieces determine one white piece. Therefore, the number of white pieces is $60 \div 3=20$ (pieces).)