The 4-digit numbers are formed from digits 0 to 9. In four digit numbers 0 is not taken at thousand’s place, so thousand’s place can be filled in 9 different ways. After filling thousand’s place, 9 digits are left. The remaining three places can be filled in

9

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P3 ways.
So the required 4-digit numbers = 9 ×

9

‌

P3 = 9 × 504 = 4536