4 digits are selected from any of the 10 numbers $0,1,2,.....9$.

Since the numbers are arranged in descending order $0$

does not come in first position.

No. of selection of any 4 numbers from $0,1,2,....9 =^{10}C_4$

Arrangement of the digits are in descending order which is only one way.

$\therefore$ The required no. of 4 digit numbers = $^{10}C_4$