Segments of your Fourth Weapon
This information is already embedded in the above walkthrough; but it's indexed
here for those who want a quick glance. (Note: this only applies to single-player
mode; for multi-player mode this information may not be correct.)
The first segment of the fourth weapon is at the end of the Seven Portals (Episode 1), in front of the portal
that leads to Episode 2.
The second segment is in one of the rooms in the Hypostyle (end of Episode 2).