4. (3 points) Given that $A, B, C, D$ and $A+C, B+C, B+D, D+A$ represent the eight natural numbers from 1 to 8, and are all distinct. If $A$ is the largest of the four numbers $A, B, C, D$, then $A$ is $\qquad$

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4. (3 points) Given that $A, B, C, D$ and $A+C, B+C, B+D, D+A$ represent the eight natural numbers from 1 to 8, and they are all distinct. If $A$ is the largest of the four numbers $A, B, C, D$, then $A$ is $\qquad$.

【Solution】Solution: $A+B+C+D+(A+C)+(B+C)+(B+D)+(D+A)=1+2+3+4+5+6+7+8$, $3(A+B+C+D)=36$,

then $A+B+C+D=12$,
$$
B+C+D=1+2+3=6,
$$

so $A=12-6=6$.
Therefore, the answer is: 6.