Reduce the word reachability problem restricted to the alphabet $\{a,b\}$ to the set of tuples $\langle u,R\rangle$ where $u$ is a word, $R$ is a word rewrite system, the used alphabet has at most $4$ symbols, and $u$ reaches $u$ using $R$ with at least one step, in order to prove that such set is undecidable (not recursive). The use of morphism is not allowed in the description of this reduction.