There seems to be a pattern here, the fourth rib is larger/longer than the third rib and it has more movement potential as well. With the increased movement potential there are increased lesions that are often issues in conditions of the heart and lungs.
The nerves associated with the fourth rib are: the fourth thoracic nerve (both anterior and posterior divisions), the fourth intercostal nerve, the fourth sympathetic ganglion, the sympathetic chain ganglia, and the recurrent meningeal nerve.
The circulatory structures associated with the fourth rib are: the fourth intercostal artery and vein, the azygos and accesory hemiazygos veins, internal mammary artery and vein, the thoracic duct, and the bronchomediastinal lymphatic ducts (as always, I may have missed something here so I urge anyone reading to dig on!).
Now that the basic structures have been outlined I will say that the most common lesion of the fourth rib is downward and inward at the sternal end with an upward movement at the vertebral end. Other lesions are possible due to increased movement potential of the fourth rib compared to those above it, but the previously noted one is most common.
Now for the juicy stuff. There is a difference in what a lesion of the right fourth rib will likely do compared to the left fourth rib! The left fourth rib will effect the heart when in lesion and the right fourth rib is more related to lung, pleural, and bronchial disorders. How can that be? Look at where the heart sits. Not only will a lesion of the fourth rib on the left side alter sympathetic tone to the heart, there is also a mechanical disturbance that will see the fourth rib place pressure on the heart altering its ability to contract. The right fourth rib lesion is more likely to have effects in lung, pleural, and bronchial disorders as it is in direct connection with those structures through nerve, artery, vein, lymphatic, and mechanical pathways.
It is important to outline the basic nature of disruptive events in a lesion of the fourth rib. Essentially there are 3 disruptive possibilities: 1 – pressure (via muscle, bone, or fluid stasis) on the nerves that connect the heart and the spinal cord leading to altered motor/trophic/sensory/vaso-motor impulses, 2- producing pressure on the fourth or fifth intercostal (Clark isn’t clear here – I read this as all of the structures in the intercostal space – nerve/artery/vein/lymphatic), and 3 – producing pressure directly on the heart and decreasing the space that it has to beat.
Knowing the basic nature of disruption, the following associated conditions may make more sense: angina pectoris, palpitations, and arrhythmia.
Other associated issues have already been touched on as relating to bronchial, pleural, and lung disorders in the same way that happens at the third rib. Mammary disorders are also associated with a lesion of the fourth rib on the same side as the mammary disorder due to the connections of nerve, artery, vein, and lymphatic structures. The fourth rib (and any rib really) carries much of the physiological traffic to and from the mammary tissue and can easily disrupt that traffic if in lesion.
A side note on correction of the fourth rib – due to the connection of serratus anterior and pectoralis minor the arm can be applied as a long lever to correct the fourth rib if done appropriately. A competent Operator should be able to apply this knowledge in all positions. Also, the respiratory pump can be very helpful in all rib lesions as the Operator can engage the internal and external intercostals to help alter the position of the rib in lesion. Knowing the theory is important, now we all have to get out there and apply it!