To escape the impending doom, they have to summon heroes from other worlds and request their aid.
... Ok. The plot's a bit overused, but since this is a retro-fantasy book, it doesn't seem to be out of place.
The four summoned Heroes all possessed their own respective, unique weapons: Sword, Spear, Bow, and Shield.
Aiya ~ to begin with, a shield can't even be considered a weapon, right? ~
I chuckled sarcastically as I continued with the story.
The heroes then ventured separately, in order to gain power in preparation against the waves of disaster.