4. A merchant accidentally mixed candies of the 1st grade (at 3 rubles per pound) and candies of the 2nd grade (at 2 rubles per pound). At what price should this mixture be sold to receive the same amount of money, given that initially the total cost of all candies of the 1st grade was equal to the total cost of all candies of the 2nd grade?
4. Let's denote by a the original cost of all candies of the 1st type. Then the total revenue from unsold candies of the 1st and 2nd type would be 2a rubles. In this case, the merchant would have $\frac{a}{3}$ pounds of candies of the 1st type, and $\frac{\text { a }}{2}$ pounds of candies of the 2nd type. Thus, for the mixture consisting of ( $\frac{a}{3}+\frac{a}{2}$ ) pounds, he should receive 2a rubles. Therefore, 
the price of the candy mixture should be equal to $\left(\frac{2 a}{\frac{a}{3}+\frac{a}{2}}\right)$ rubles. We get that $\frac{2 a}{\frac{a}{3}+\frac{a}{2}}=\frac{2 a}{\frac{2 a+3 a}{6}}=\frac{12 a}{5 a}=2.4$ Therefore, the candy mixture should be sold at 2 rubles and 40 kopecks per pound.

Answer: 2 rubles 40 kopecks.