4. There is a kind of football made of 32 pieces of black and white cowhide, with black pieces being regular pentagons and white pieces being regular hexagons, and all sides being of equal length (as shown in Figure 2). Then the number of white pieces is ( ).
(A) 22
(B) 20
(C) 18
(D) 16
4.B.

Let the number of white pieces be $x$, then the number of black pieces is $32-x$.
Since the black pieces are pentagons, the total number of edges of the black pieces is $5(32-x)$.

Also, because each white piece has 3 edges connected to black pieces, the total number of edges of the black pieces can also be expressed as $3x$, leading to the equation
$$
5(32-x)=3 x \text {. }
$$

Solving this, we get $x=20$.