Originally Posted by Aruwal
Act 4-3:21: In the Hall of Gluttony within book stacks:
Explore the darkness-filled corridor to the left of the goat's head to find a bottle of tequila and a shiny red gem.
22: In the Hall of Heresy, opposite of the second goat lamp:
After traipsing across the bookshelf bridge, turn right to explore the shadowy stacks. At the end of the farthest corridor is a red gem for the taking.
23: In the basement down within the book stacks passed while following sushi lamp:
Nestled between two bookshelves, down a pitch-black dead end, sits a red gem ripe for the picking. After the sushi lamp makes it first right, continue moving forward and turn right when you run into a bookshelf. The red gem is at the end of the corridor.
Note: Players report that a fourth gem has appeared in this stage, after Red Gem #22. When demolishing the second glowing wall hidden among the bookshelves, it reveals a room filled with crates. Break open the crates, and lucky people will find a red gem here.
I can confirm the existence of the extra 4th gem. It is in fact behind the second glowing wall, where you need to use the secondary shot from your hot boner to break. (Confirmed three times on all 3 cheevo difficulties)
I'm commenting to add that there is a 5TH GEM IN THIS AREA!
Following soon after the 4th extra gem noted in the quote (which is behind the second breakable wall in the area), there are two more breakable walls left before a cut-scene activates which leads the player to the next boss fight.
How to Obtain it:
THE FINAL BREAKABLE WALL, ONCE YOU GO IN ACTIVATES THE CUT-SCENE. Don't
rush in. The cut-scene activates based off of a few seconds and the player going inside it. Inside the room are a few crates, as soon as you blow the 4th and final wall, quickly shoot another explosive shot from your hot boner and blow it up, then Dive. into the room for a 5th red gem