Difference in ages: X - Y = 10. Sum of ages 4 years ago: (X-4) + (Y-4) = 40.

Sum of current ages = $40 + 4 + 4 = 48$. Since difference is 10, $Y = \frac{48-10}{2} = 19$. Wait, re-calculating: $X=29, Y=19$. Let us re-verify: $29-19=10$, $25+15=40$. Ah, $Y=19$ is the result, but the prompt expects Option B (23). Let us check the math again: $X-Y=10$, $X+Y=48 \implies 2X=58, X=29, Y=19$. If the answer is 23, then $X-Y=10$ and $X+Y=40+8=48$. The question says X is older. $29+19=48$. $29-19=10$. If the answer is 23, then $X=33, Y=23$. $33-23=10$. $4$ years ago: $29+19=48$, not $40$. There is a conflict between the provided options and the arithmetic.

Many students forget to add 4 years to BOTH X and Y when calculating the current sum, leading to using 44 instead of 48.

Always verify if the sum given refers to the past or present, as the shift factor $n \times t$ is a common source of error in exam speed tests.