Irreducibility and Monodromy of Automorphic Galois Representations of $\mathrm{G}\mathrm{L}(4)$

We prove that over totally real fields, the $p$-adic Galois representations attached to non-self-dual regular algebraic cuspidal automorphic representations of $\mathrm{G}\mathrm{L}(4)$ are irreducible. We then develop the theory of extra-twists in a general setting and use it to compute the monodromy group (over $\mathbb{Q}$) of these Galois representations, in both self-dual and non-self-dual settings, and prove $p$-adic and residual big image results.