  Motivated by M. Scharlemann and A. Thompson's definition of thin position of
3-manifolds, we define the width of a handle decomposition a 4-manifold and
introduce the notion of thin position of a compact smooth 4-manifold. We
determine all manifolds having width equal to $\{1,\dots, 1\}$, and give a
relation between the width of $M$ and its double $M\cup_{id_\partial} \overline
M$. In particular, we describe how to obtain genus $2g+2$ and $g+2$ trisection
diagrams for sphere bundles over orientable and non-orientable surfaces of
genus $g$, respectively. By last, we study the problem of describing relative
handlebodies as cyclic covers of 4-space branched along knotted surfaces from
the width perspective.
