Answer 1

Disclaimer:- (1) Here, we can form 4 digits, 5 digits , 6 digits numbers and so on.... using the given digits. Thus, infinite numbers can be formed.
(2) Taking into account only four digit numbers.
We have to find all the numbers that can be formed by using the digits 1, 2, 3 and 4. This means that repetition of digits is not allowed as all the digits have to be used.
Total numbers that can be formed = Number of arrangements of four digits, taken all at a time =  4! = 24