  Within crystallization theory, two interesting PL invariants for
$d$-manifolds have been introduced and studied, namely {\it gem-complexity} and
{\it regular genus}. In the present paper we prove that, for any closed
connected PL $4$-manifold $M$, its gem-complexity $\mathit{k}(M)$ and its
regular genus $ \mathcal G(M)$ satisfy: $$\mathit{k}(M) \ \geq \ 3 \chi (M) +
10m -6 \ \ \ \text{and} \ \ \ \mathcal G(M) \ \geq \ 2 \chi (M) + 5m -4,$$
where $rk(\pi_1(M))=m.$ These lower bounds enable to strictly improve
previously known estimations for regular genus and gem-complexity of product
4-manifolds. Moreover, the class of {\it semi-simple crystallizations} is
introduced, so that the represented PL 4-manifolds attain the above lower
bounds. The additivity of both gem-complexity and regular genus with respect to
connected sum is also proved for such a class of PL 4-manifolds, which
comprehends all ones of "standard type", involved in existing crystallization
catalogues, and their connected sums.
