7. (10 points) Fill in each cell with a number from $1-6$, so that the numbers in each row, each column, and each $2 \times 3$ rectangle (enclosed by thick lines) are not repeated; if the sum of the numbers in the cells on both sides of a small circle is a composite number, and the sum of other adjacent cells is a prime number, then the four-digit number “相约华杯” is 4123. Because the third row contains $1, 3, 4$, $A$ must be one of $2, 5, 6$, and since the sum of 3 and $A$ is a prime number, $A$ is 2. In the rectangle where $A$ is located, the numbers 1, 4, 5, 6 are still unused. Since the sum of 3 and “相” is a prime number, “相” is 4. The sum of “相” and “约” is a prime number, so “约” is 1. The sum of “约” and “月” is a prime number, so “月” is 6, and the remaining $C$ is 5. The third row has only the number 5 left, so $B$ is 5; in the rectangle where $B$ is located, the numbers $2, 3, 6$ are still unused. Since the sum of 4 and “杯” is a prime number, “杯” is 3, and the sum of “杯” and “华” is a prime number, so “华” is 2, and the remaining $D$ is 6; 

So the four-digit number “相约华杯” is 4123.
Therefore, the answer is: 4123.
7. (10 points) Fill in each cell with a number from $1-6$, so that the numbers in each row, each column, and each $2 \times 3$ rectangle (enclosed by thick lines) do not repeat; if the sum of the numbers in the cells on both sides of a small circle is a composite number, and the sum of the numbers in other adjacent cells is a prime number, then the four-digit number “相约华杯” is $\qquad$ .

Note: The phrase “相约华杯” is a Chinese phrase and does not have a direct English translation. It can be left as is or transliterated as “Xiangyue Huabei”.