Ari. Nice to meet you.
Well, the "average" party (assuming such an animal exists) often consists of one character to fill each of the primary adventuring niches. That is, you likely have one warrior-type (fighter, barbarian, etc.), one arcanist (wizard, sorcerer), one divine caster (cleric, druid), and one rogue type (rogue). This isn't a requirement or a rule, of course, but it's sort of the default assumption.
Certain other classes
can fill those roles, but are actually more effective in parties of more than four. For instance, if you add a bard to the above list, he can either stand in as backup for one of the primary classes, or he can do his own thing and play to his own strengths.
That's where I see the shadowcaster. With the right choice of mysteries, he can serve as a scout or a rogue. With a slightly different choice, he can function as a "blaster." He may not be as good at either as a true rogue or sorcerer, but he
can accomplish either function.
That's what I mean by a "5th character." Someone who may not fill any of the primary niches
as well as a standard class, but who can instead fill multiple niches at need.