I have never had the benefit of a 5th player but there are a number of the more difficult quests where even an extra meat shield won't be much help without serious tactics on the players side. Especially if they have not geared up, or have poorly geared.
A second Wizard may seem OP....but have you ever played a game where the wizard has had every combat spell fail and the Elf rolls nothing on the Genie? Sucks hard. Just happened to me last game. (The Lost Wizard is a hard one with limited gear.) While it got pulled off my Dwarf almost bit the dust twice and used up every heal potion discovered to date. Going into the Fire Mage with no heals other than the 2 heal spells the wizard will have. A 5th player would be very welcome in such instances.
Be glad when they get this App filled out with all the quests for all the books. I like it how it is giving me a first hand view of certain quest difficulty. I may have been a little brutal with how many Champion level monsters I place in the game. (Champion Monster at my table means they defend on White Shields.) But when 2 party members have cross bows, and they are good at placement and tactics it feels apt.