4. A, B, and C buy study materials together. It is known that A and B paid a total of 67 yuan, B and C paid a total of 64 yuan, and A and C paid a total of 63 yuan. Therefore, A paid $\qquad$ yuan.
【Analysis】A and B paid a total of 67 yuan, B and C paid a total of 64 yuan, and A and C paid a total of 63 yuan. Adding these three amounts together gives twice the total amount paid by the three people. Dividing this by 2 gives the total amount paid by the three people. Subtracting the amount paid by B and C from this total gives the amount paid by A.

【Solution】Solution: $(67+64+63) \div 2$
$$
\begin{array}{l}
=194 \div 2 \\
=97 \text { (yuan) }
\end{array}
$$
$$
97-64=33 \text { (yuan) }
$$

Answer: A paid 33 yuan.
Therefore, the answer is: 33.