Question

Any number that is divisible by 4 is also divisible by 8. Find a counterexample to show that the conjecture is false

Answer 1

If anythings that is divisible by 4 and by 8 then 12 would be a example to prove it false

Answer 2

' 4 ' itself is a fine counterexample. 
4 is divisible by  4  but not by  8 .

Any odd-multiple of  4  is another counterexample. 
In other words, only even-multiples of  4  are divisible by  8 .