Existence of multi-bump solutions to biharmonic operator with critical exponential growth in ${\mathbb{R}}^4$

Using variational methods, we establish existence of multi-bump solutions for the following class of problems $$\{\begin{array}{l}{\mathrm{\Delta }}^{2}u+(\lambda V(x)+1)u=f(u),in{\mathbb{R}}^4,u\in H^2({\mathbb{R}}^4),\end{array}$$ where ${\mathrm{\Delta }}^2$ is the biharmonic operator, $f$ is a continuous function with critical exponential growth and $V:{\mathbb{R}}^4\to \mathbb{R}$ is a continuous function verifying some conditions.