In the room that Trish and Eldridge are in you'll find a wall with a large crack in it- throw a stick of dynamite at it to get a piece of parchment. Once you leave the room toss another dynamite stick at the wall adjacent to the door you just left- you'll get the yellow crystal and a save token. Head back downstairs at this point.
In the Colonial Area throw a stick of dynamite at the door with a big white sticker on it. You'll get another crystal- head upstairs again. The third crystal is behind the section of wall below the Roaring Bear- one more stick of dynamite here. Last one is in the Volcanic area- throw some dynamite into the center of the Earth's Core display.
In this new room, lies a Spell "Summon" which summons a hell hound that helps you attack, a mana upgrade, and the Launcher (behind the benches). Grab em and go back to the "eldridge and Trish" Room. Once in the room, place the crystals in the door in this order. Red, yellow, blue, green. Once inside, make a right at the main rooms door and grab a save token, then head on in. Keep on going, make sure to grab the save token in the middle of the giant room with staircases on both sides, asend the stair cases (being sure to grab the medipack on top, inbetween both staircases) and head in the room.