They tell us very different information:
V4 lead placement is on the left side of the chest, 5th intercostal space, midclavicular line and observes the anterior wall of the left ventricle. Whereas V4R is in on the RIGHT side of the chest, 5th intercostal space, midclavicular line and observes the right ventricle.
Remember that V4R application is performed to rule in/out a right ventricular MI, in the setting of an inferior STEMI. These patients are often preload dependent and administration of nitroglycerin may cause significant hypotension.
Therefore, if ST-elevation > 1mm in V4R, withhold nitroglycerin.
Figure Modified from: ecgwaves.com