Option 1 refers to the difference in the ratio of males to females in a population, which may reflect a gender imbalance. This can happen due to various factors such as higher mortality rates for one gender or gender-specific migration patterns. However, the term demographic gap does not specifically refer to this.

Option 2 refers to the difference in age distribution within a population. This can indicate variations in birth rates, death rates, or migration patterns. While this is related to demographics, it is not the precise definition of the term demographic gap.

Option 3, the child/woman ratio, refers to the number of children in a population per woman of reproductive age. This ratio is often used as an indicator of fertility levels within a population. However, it is not the precise definition of the term demographic gap.

Option 4 correctly identifies the meaning of the term demographic gap. It refers to the difference between the birth rate and the death rate within a population. This difference indicates the overall growth or decline of a population. A positive demographic gap indicates population growth, while a negative gap indicates population decline.

Therefore, option 4 is the correct answer as it accurately defines the term demographic gap.