Let
$X_4\subset \mathbb {P}^{n+1}$
be a quartic hypersurface of dimension
$n\geq 4$
over an infinite field k. We show that if either
$X_4$
contains a linear subspace
$\Lambda $
of dimension
$h\geq \max \{2,\dim (\Lambda \cap \operatorname {\mathrm {Sing}}(X_4))+2\}$
or has double points along a linear subspace of dimension
$h\geq 3$
, a smooth k-rational point and is otherwise general, then
$X_4$
is unirational over k. This improves previous results by A. Predonzan and J. Harris, B. Mazur and R. Pandharipande for quartics. We also provide a density result for the k-rational points of quartic
$3$
-folds with a double plane over a number field, and several unirationality results for quintic hypersurfaces over a
$C_r$
field.