"Traditionally, a 24-hour day is divided into seven watches. These are: midnight to 4 a.m. [0000-0400], the mid-watch; 4 to 8 a.m. [0400-0800], morning watch; 8 a.m. to noon [0800-1200], forenoon watch; noon to 4 p.m. [1200-1600], afternoon watch; 4 to 6 p.m. [1600-1800] first dog watch; 6 to 8 p.m. [1800-2000], second dog watch; and, 8 p.m. to midnight [2000-2400], evening watch. The half hours of the watch are marked by the striking the bell an appropriate number of times."
from the United States Navy's site
Origins of Navy Terminology[/color]
Therefore, four bells would indicate two hours into a watch. This would be the middle of most watches, but the end of a dog watch. Therefore, eight bells would be the end of any watch other than a dog watch.