These trim little sandwiches are an elegant picnic food. Pack them snugly into a box lined with a tea towel spritzed with water so they won't dry out.
Combine the tuna and mayonnaise in a medium bowl. Spread tuna mixture on 4 slices of bread. Spread tapenade on remaining 4 slices of bread; place over slices with tuna mixture, tapenade side down. Trim crusts; cut sandwiches in half.