  We directly apply the theory of viscosity solutions to partial differential
equations of order greater than two. We prove that there exists a solution in
$C^{2,\alpha}(B_R)\cap C(\overline{B_R})$ for the inhomogeneous
$\infty$-Bilaplacian equation on a ball $B_R\subset \mathbb{R}^n$:
$$\Delta_\infty^2 u:=(\Delta u)^3 |D(\Delta u)|^2 =f(x)$$ with Navier Boundary
conditions ($u=g\in C(\partial B_R), \Delta u =0 \textrm{ on } \partial B_R$).
We also prove that there exists a solution in $C^{1,\alpha}(\mathbb{R}^n)$ for
all $\alpha>0$ to the eigenvalue problem on $\mathbb{R}^n$: $$\Delta_\infty^2 u
=-\lambda u+f(x)$$ whenever $n\geq 3, \lambda<0,$ and $f(x)$ is continuous,
bounded, and supported on an annulus.
