11. The figure below is a $9 \times 9$ grid, divided by thick lines into 9 smaller $3 \times 3$ grids, each containing 3 rows and 3 columns of cells. Some of the cells are filled with numbers from 1 to 9. Xiao Qing filled in the empty cells in the 4th column with natural numbers from 1 to 9, ensuring that no number is repeated in any row, column, or $3 \times 3$ grid. Then, Xiao Qing wrote down the 9-digit number formed by the numbers in the 4th column from top to bottom. Please write down this 9-digit number and briefly explain the reasoning.
11. Solution:

Use $(\mathrm{a}, \mathrm{b})$ to denote the square at the $\mathrm{a}$-th row and the $\mathrm{b}$-th column. The 4th column already has the numbers $1, 2, 3, 4, 5$, and the 6th row already has the numbers $6, 7, 9$, so the square $(6,4)=8$; The 3rd and 5th rows both have the number 9, so $(7,4)=9$; The central "small 3x3 grid" already has the number 7, so it can only be $(3,4)=7$; At this point, only $(5,4)$ remains in the 4th column, and the only number missing in this column is 6, so $(5,4)=6$. Therefore, the 9-digit number formed by the numbers in the 4th column from top to bottom is: 327468951.