  We are concerned on the fourth-order elliptic equation
\begin{equation}\tag{$P_\lambda$} \left\{ \begin{array}[c]{ll} \Delta^2 u-
\Delta u + V(x)u -\lambda \Delta[\rho(u^2)]\rho'(u^2)u= f(u)\, \, \mbox{in} \,
\, \mathbb{R}^N, & u\in W^{2,2}(\mathbb{R}^N), \end{array} \right.
\end{equation} where $\Delta^2 = \Delta(\Delta)$ is the biharmonic operator,
$3\leq N\leq 6$, the radially symmetric potential $V$ may change sign and
$\inf_{\mathbb{R}^N}V(x)=-\infty$ is allowed. If $f$ satisfies a type of
nonquadracity and monotonicity conditions and $\rho$ is a suitable smooth
function, we prove, via variational approach, the existence of a radially
symmetric nontrivial ground state solution $u_\lambda$ for problem
$(P_\lambda)$ for all $\lambda\geq 0$.
