Maiwa sold 15 cups of smoothies on the first day. Each day, she sold 8 more cups than the previous day. How many cups did Maiwa sell on the fourth day?

The number of cups sold on the first day is the first term, $a_1$.
The increase in cups sold each subsequent day is the common difference, $d$.

$$a_1 = 15 \text{ cups}$$ $$d = 8 \text{ cups}$$

The formula for the $n$-th term is $a_n = a_1 + (n-1)d$. We need to find the number of cups sold on the fourth day, so $n=4$.

$$a_4 = a_1 + (4-1)d$$ $$a_4 = 15 + (3) \times 8$$ $$a_4 = 15 + 24$$ $$a_4 = 39$$

Maiwa sold 39 cups on the fourth day.

The final answer is $\text{39 cups}$.