4. There are 25 cards numbered $1, 3, \cdots, 49$. If a card with number $a$ is drawn, the next card to be drawn is the card with the largest odd divisor of $99-a$. This process is repeated until no more cards can be drawn. Xiao Hua drew the card numbered 1. When the operation ends, how many cards are left?
(A) 9
(B) 10
(C) 16
(D) 17
4. C.

The card numbers drawn are
$$
1 \rightarrow 49 \rightarrow 25 \rightarrow 37 \rightarrow 31 \rightarrow 17 \rightarrow 41 \rightarrow 29 \rightarrow 35 \rightarrow 1 \text {. }
$$

A total of 9 cards were drawn, leaving 16 cards.