Two answers are possible.
One is with the repetition of digits.
The other is without repetition of digits.
ANSWER 1: With Repetition
Numbers: 1,2,4,5,7,8
There are 4 digits __ __ __ __
1st digit can be filled by any of the 6 numbers.
Similarly 2nd, 3rd and 4th digits.
Hence each digit will have 6 possibilities.
Therefore no.of 4 digit numbers that can be formed are 6 x 6 x 6 x 6 = 1296
ANSWER 2: Without Repetition
4 digits __ __ __ __
Now the first digit can be filled by any of the six numbers. Therefore there are 6 possibilities
The second digit will have only 5 possibilities as one of the numbers gets used up by the 1st digit.
Similarly the 3rd and 4th digits will have 4 and 3 possibilities respectively.
Therefore no.of 4 digit numbers that can be formed are 6 x 5 x 4 x 3 = 360