Define $3$Reachability as the following:

$3$Reachability: Given nodes n and n$\text{'}$ in a directed graph, is there a finite path of transitions from n to n$\text{'},$ of length greater than $3?$

Prove that $3$Reachability is not expressible in Predicate Logic. To solve this problem you can use either lowenheim$-$skolem theorem or reachability is not expressible in predicate logic or compactness theorem