  We prove a Brezis-Kato-type regularity result for weak solutions to the
biharmonic nonlinear equation $$ \Delta^2 u = g(x,u)\qquad\text{in
}\mathbb{R}^N$$ with a Carath\'eodory function $g:\mathbb{R}^N\times
\mathbb{R}\to \mathbb{R}$, $N\geq 5$. The regularity results give rise to the
existence of ground state solutions provided that $g$ has a general subcritical
growth at infinity. We also conceive a new biharmonic logarithmic Sobolev
inequality $$ \int_{\mathbb{R}^N}|u|^2\log |u|\,dx\leq\frac{N}{8}\log
\Big(C\int_{\mathbb{R}^N}|\Delta u|^2\,dx \Big), \quad\text{for } u \in
H^2(\mathbb{R}^N), \; \int_{\mathbb{R}^N}u^2\,dx = 1, $$ for a constant $0<C<
\big(\frac{2}{\pi e N}\big)^2$ and we characterize its minimizers.
