I am frequently left with four egg whites in a jar, after I've made a pint of custard, which uses four egg yolks. Meringues are the quickest and simplest thing I know to use up egg whites.
Turn on your oven to 100°C and line oven trays with silicone sheets. For each egg white you need 50 g sugar. Whip the egg whites until stiff then slowly pour in the sugar whilst still beating. The whole process should take 2-3 minutes. Using spoons, place dollops of meringue on the oven trays, pulling them up so they form attractive peaks. Put the meringues in the oven for 40 minutes. Check after 20-30 minutes and if they are starting to brown open the oven door for a minute to cool it down. At the end of the cooking time turn the oven off, but leave the meringues in there until the oven is cold (many people leave meringues in overnight, which works well).
Four egg whites will make about 30 meringues.