I assume $J$ is the set of integers and $J_6$ is the ring of integers modulo 6. (My first group theory text used $J$ to denote the integers.) In $J_6$, 2 divides 4 ($4=2\cdot2$) and 4 divides 2 ($2=4\cdot 5$). So 2 is an associate of 4. Just test all remaining non-zero elements of $J_6$.