Aren't there monster tokens for all of the villains (in all 3 games) as well? That amps up the tension if some of those are in the deck.
Additional random villains could be a bit too much in an otherwise balanced scenario. However if you use additional levelling rules or otherwise carry half of the treasure deck on your heroes, it could be a good way to make things hard again.
As I think that the four monster token makes things too fiddly and brutal (if one of the monsters is a legion devil, you could be facing 6 monsters that would activate before you), I've sometimes used it to mark the appearance of a special monster, a miniboss if you will. For example the special monster could be a BGG-made four experience monster, that does not have a proper plastic card, and thus cannot be spawned via the regular monster deck.