4. (5 points) This year, the sister's age is 3 times the younger sister's age, in 2 years, the sister's age will be 2 times the younger sister's age, so, this year, the sister's age is $\qquad$ years.

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【Analysis】Let the sister's age this year be $x$ years old, then according to “this year, the older sister's age is 3 times the younger sister's age,” we know that the older sister's age this year is $3 x$ years old. In 2 years, the younger sister's age will be $x+2$ years old, and the older sister will be $3 x+2$ years old. Finally, according to “in 2 years, the older sister's age will be twice the younger sister's age,” which means in 2 years, the older sister's age $=$ the younger sister's age $\times 2$, we can set up an equation to find the younger sister's age, and then find the older sister's age.

【Solution】Solution: Let the younger sister's age this year be $x$ years old, then the older sister's age this year is $3 x$ years old, so in 2 years, the younger sister's age will be $x+2$ years old, and the older sister will be $3 x+2$ years old,
$$
\begin{aligned}
3 x+2 & =2 \times(x+2), \\
3 x+2 & =2 x+4, \\
x & =4-2, \\
x & =2,
\end{aligned}
$$

The older sister's age this year: $3 \times 2=6$ (years old),
Answer: This year, the older sister's age is 6 years old,
Therefore, the answer is: 6.