Thousands’ place (10³) can be filled with 1, 2, 3 and 4 i.e. thousands’ place can be filled in 4 ways.

Hundreds’ place (10²) can be filled in 3 ways [i.e. with the remaining 3 digits].

Similarly, tens’ place (10¹) can be filled in 2 ways and ones’ place can be filled only in 1 way.

As, the operations are dependent, so, total number of ways

=24

The discussion can be shown pictorially as:

ALTERNATIVE APPROACH:

The number of permutations of 4 objects taken 4 at a time is = ⁴P₄ = 4! = 24.