  We obtain an improved Kakeya maximal function estimate in $\mathbb{R}^4$
using a new geometric argument called the planebrush. A planebrush is a higher
dimensional analogue of Wolff's hairbrush, which gives effective control on the
size of Besicovitch sets when the lines through a typical point concentrate
into a plane. When Besicovitch sets do not have this property, the existing
trilinear estimates of Guth-Zahl can be used to bound the size of a Besicovitch
set. In particular, we establish a maximal function estimate in $\mathbb{R}^4$
at dimension $3.059$. As a consequence, every Besicovitch set in $\mathbb{R}^4$
must have Hausdorff dimension at least $3.059$.
