Egg whites: I normally have some egg whites left in the fridge and normally work on 1 egg white = 30g, so 4 egg whites will be 120g.
Always ensure a clean bowl when adding the egg whites. making sure there is no shells in the mixture or the whites will not whip to stiff peaks.
Stiff Peaks: to test whether your eggs have reached stiff peaks, you should be able to hold the bowl upside down with no movement of the egg white mixture.
When cooking with eggs the low temperature of the oven is really important and this is where an oven thermometer will be really handy.