A deliverable has been defined as occurring at the fourth level of the Product Breakdown Structure. Which is the correct numbering?

$1.$

$1\mathrm{.}1\mathrm{.}1\mathrm{.}1$

$2.$

$1\mathrm{.}4\mathrm{.}1\mathrm{.}1$

$3.$

$4\mathrm{.}1\mathrm{.}1\mathrm{.}1$

$4.$

$1\mathrm{.}1\mathrm{.}1\mathrm{.}4$

$5.$

$1\mathrm{.}4$