5th member not really needed.
The "need" is irrelevant.
People can finish the game even playing solo if they try hard enough.
It doesn't change the fact that a larger party offers its own benefits in terms of composition versatility and they aren't necessarily tied to the difficulty.
I think the encounters in D&D have been based around a party of four since 3.5.
This is irrelevant as well.
Not to mention it's a thing for entirely different reasons: gathering multiple people around a table is hard to coordinate with friends, so the lower they set the basic requirements for a group, the better it is.
This is a logistic obstacle that simply doesn't exist in a CRPG.
