  In this paper, we study $n$-dimensional complete minimal hypersurfaces in a
hyperbolic space $H^{n+1}(-1)$ of constant curvature $-1$. We prove that a
$3$-dimensional complete minimal hypersurface with constant scalar curvature in
$H^{4}(-1)$ satisfies $S\leq \frac{21}{29}$ by making use of the Generalized
Maximum Principle, where $S$ denotes the squared norm of the second fundamental
form of the hypersurface.
