High calorie snacks are not going to help and may hinder their development and capacity to produce eggs. Stick with a balanced diet as
@sourland suggests. With layer feed(15-16% protein) keep treats to a bare minimum(less than 10% of daily ration volume) so as not to dilute the protein and other nutrients in feed.
2 should be plenty for 4 birds, but 1 more might not hurt.
If another nest is taking up floor space, best to not add it. Nest should be lower than roosts. How big is your coop, in feet by feet? Pics would
really help.