4. (5 points) When Xiaohong was 3 years old, her mother's age was the same as Xiaohong's age this year; when her mother is 78 years old, Xiaohong's age will be the same as her mother's age this year. How old is Xiaohong's mother this year.
53
【Analysis】Let the age difference between mom and Xiaohong be $x$ years, then according to “When Xiaohong was 3 years old, mom's age was the same as Xiaohong's age this year;” we get Xiaohong's age this year is: $x+3$ years; According to “When mom is 78 years old, Xiaohong's age is the same as mom's age this year,” we get Xiaohong's current age is: $78-x$ years; According to Xiaohong's age + age difference = mom's age, we can set up the equation to solve the problem.

【Solution】Solution: Let the age difference between mom and Xiaohong be $x$ years, then Xiaohong's current age is $x+3$ years, and mom's current age is $78-x$ years. According to the problem, we can set up the equation:
$$
\begin{aligned}
x+3+x & =78-x \\
2 x+3 & =78-x \\
2 x+x & =78-3 \\
3 x & =75 \\
x & =25 \\
78-25 & =53 \text { (years) }
\end{aligned}
$$

Answer: Mom is 53 years old this year.
The answer is: 53.