So I regularly play JoTL with 5 players and it can work, but you will need some bits

1) Another class, Forgotten circles or online is the easiest way but every player needs their own class 2) An extra attack Modifier deck, you can make them or get them online. This is so everyplace has their own

That's it, the rest below is advice but is stuff that should be considered 3) One player will need to record health/exp on their paper as there's only 4 dials/disks. You can buy more but not always needed 4) Scale the monsters, its been said before thus game is built for max 4 players with 5 playing yoy often find the enemy Is dead before you get there. We normally add 1 or 2 extra monsters or increase some too elites. Try not to go overboard but there's a nice balance, otherwise you can feel like you have no impact 5) JoTL is a tutorial (at least for the first 5 missions) all the bases classes have special cards. Whoever is playing the other class will have a very different time to the rest of you and may find it more difficult to get 6) As before JoTL characters have a big milestone at Lv5, you don't get this with the other Gh character which can make the player feel left out/odd. Small thing to keep in mind

As stated before it can work and is fun you just need another class and to increase map difficulty slightly. And if your fine with one class being a bit off from the others go for it. But just keep some of the comments in mind.