8 We notice that $18^{2}=324, 24^{2}=576$, which are formed by the three consecutive digits $2,3,4$ and $5,6,7$ respectively, after appropriate rearrangement; and $66^{2}=4356$ is formed by the four consecutive digits $3,4,5,6$ after appropriate rearrangement; then, the next such square number is: $\qquad$
8 $74^{2}=5476$. Hint: For any square number, its last digit can only be one of $0,1,4,5,6,9$; since $(10 a)^{2}$ ends with two 0s; if the last digit of a square number is 5, then its second last digit is 2; if its last digit is 6, then its second last digit is odd; if its last digit is $1,4,9$, then its second last digit is even. This is due to:
$$
\begin{array}{l}
(10 a+5)^{2}=100 a^{2}+100 a+25, \\
(10 a \pm 4)^{2}=100 a^{2} \pm 80 a+16, \\
(10 a \pm 1)^{2}=100 a^{2} \pm 20 a+1, \\
(10 a \pm 3)^{2}=100 a^{2} \pm 60 a+9, \\
(10 a \pm 2)^{2}=100 a^{2} \pm 40 a+4 .
\end{array}
$$

If 3, 4, 5, 6 can form other square numbers, according to the above rules, they can only be $3456,4536,5436,5364,3564$, but none of them are square numbers; the next four consecutive digits are $4,5,6,7$, if they are to form a square number, the last digit can only be 4 or 6, from smallest to largest they are $4576,4756,5476,5764,7456,7564$, only $5476=$ $74^{2}$ is a square number.