4. (8 points) A furnace of molten iron solidifies into an iron block, its volume decreases by $\frac{1}{34}$; then, if this iron block melts back into molten iron (without any loss), its volume increases by $\frac{(}{()}$.
【Answer】Solution: The volume of the iron block is that of the molten iron: $1-\frac{1}{34}=\frac{33}{34}$, the volume of the iron block increases when it melts into molten iron: (1- $\left.\frac{33}{34}\right) \div \frac{33}{34}=\frac{1}{33}$. Answer: If this iron block melts back into molten iron (without any loss), its volume increases by $\frac{1}{33}$. Therefore, the answer is: $\frac{1}{33}$.