First, you need to get the amount of digits in the number. Since it says "Enter a 4 digit number", I guess you can assume it always have 4 digits (in the other cases it will be undefined).
Second, use a for loop and the modulus operator. N mod 10 (N % 10, expressed in C code) gives the last digit in N. You want to multiply this number with 1000. Then you want to get the second last digit, which you get by doing a truncating division by 10 then using modulus again. Multiply this number with 100 and add it to your last result. Repeat this a total of 4 times.
By gradually multiplying the result by 10 instead of 1000 (the first time) you can do this algorithm with a variable number of digits, not just 4. See the code below:
int ReverseDigits(int Number)
int Result = 0;
while(Number > 0)
Result *= 10;
Result += Number % 10;
Number /= 10;