The value 12 is a good counterexample. It is divisible by 4 (since 12/4 = 3 is a whole number with no remainders or decimal portions) but 12 is not divisible by 8 (note how 12/8 = 1 remainder 4 = 1.5)
To find other examples, add 8 to 12 to get 20. Then add on 8 more to get 28. This pattern continues on forever.