4. A bookstore is holding a summer book fair, where book buyers can enjoy the following discounts:
(1) For a single purchase not exceeding 50 yuan, no discount is given;
(2) For a single purchase exceeding 50 yuan but not exceeding 200 yuan, a 10% discount is given on the marked price;
(3) For a single purchase exceeding 200 yuan, the first 200 yuan is discounted according to rule (2), and the amount exceeding 200 yuan is given an 80% discount.

A student made two purchases, paying 81 yuan and 126 yuan respectively. If he had bought the same books in one go, he would have saved $\qquad$ yuan.
4.3.

Obviously, 81 and 126 are both the prices after the discount. If not discounted, the combined price should be
$$
(81+126) \div 0.9=230 \text { (yuan). }
$$

Since 230 yuan exceeds 200 yuan, according to (3), it should be
$$
200 \times 0.9+30 \times 0.8=204 \text { (yuan) } \text {. }
$$

Thus, we have $81+126-204=3$ (yuan).