Ethane is a symmetric molecule, therefore both groups of hydrogens occur in the same region and not in two different regions (for example chloroethane would have two different regions). Using the n+1 rule, the hydrogens on one carbon couple to the 3 hydrogens on the adjacent carbon. If n=3 therefore 3+1=4 peaks would be seen. On the NMR spectra of Ethane you would only see a quartet.