Spread one side of each slice of bread with a thin layer of butter. Arrange a single layer of cucumber slices on 4 of the bread slices. Stir together the mayonnaise, chives, mustard and some salt and pepper in a small bowl. Spread a thin layer of the mayonnaise mixture over the cucumbers. Top with the remaining slices of buttered bread. Cut off the crusts and cut each sandwich into three fingers.