The fourth thoracic ventral rami pass into the intercostal space between the pleura and internal intercostal membrane (fascia).  They then pass below the intercostal vessels in the neurovascular plane between the fourth internal and innermost intercostal muscles below the intercostal vessels.  Before reaching the costal angle they give rise to lateral and collateral branches.

The fourth lateral cutaneous branch pierces the intercostal muscles obliquely, close to the midaxillary line before dividing into anterior and posterior branches.  The anterior branch runs forwards over pectoralis major, while the posterior passes back over the scapula and latissimus dorsi to terminate in the overlying skin.  

The fourth anterior cutaneous branch is the terminal part of the collateral nerve.  The collateral nerve follows the inferior border of its intercostal space to pierce the internal intercostal muscle, external intercostal membrane and the pectoralis major muscle near the sternum where it becomes the fourth anterior cutaneous branch of the thorax and terminates in the overlying skin.  

Supply
The fourth lateral cutaneous branch supplies skin over the fourth intercostal space and adjacent parts of the third and fifth intercostal spaces.  The anterior cutaneous branches supply skin over the anterior thoracic wall of the fourth intercostal spaces.