A number is divisible by 3 if its digits add up to a number divisible by 3. We can tell that 13752 is divisible by 3 because 1 + 3 + 7 + 5 + 2 = 18 and 18 is divisible by 3.
A number is divisible by 6 if it is divisible by 3 and divisible by 2. A number is divisible by 2 if the last digit on the right is even. We already know 13752 is divisible by 3, but we also know that 13752 is divisible by 6 because it ends in a 2 and 2 is even.
We want a 4 digit number that is divisible by 3 but not 6.
Let's try 3333.
Is 3333 divisible by 3?
3 + 3 + 3 + 3 = 12, 12 is divisible by 3, so yes, 3333 is divisible by 3.
Is 3333 divisible by 6?
We already know 3333 is divisible by 3, but the last digit is a 3 (odd), so 3333 is not divisible by 6.
Answer: (one possible answer) 3333