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Right, since you have the Blue Gem, from the Main Hall, head to the Dining Room, then into Kenneth's Corridor, and then into the West Corridor (where you can access the Greenhouse and the Keeper's Room). There's a room accessible from this corridor which is very small and has a statue of a tiger with the inscription "Some tigers have a blue eye and a red eye", or something. Use the Blue Gem. Jill will place it in one of the tiger's eye holes and you'll receive one of the Crests. Remember to come back here when you obtain the Red Gem, because when you use it here you'll be given the Magnum. This is later on in the game when you return to the Mansion, though.

Head to the Researcher's Room. It is a room accessible in the corridor above the East Save Room. Inside is a fish tank. Draining it, moving it and then moving the bookcase should get you another Crest.

In the gallery with the pictures, you have to press all the switches in a certain order - as it says, from cradle to the grave. This means that you have to start with the picture of the youngest person and end with the picture of the oldest person. Here is the order:

Newborn baby - 1st
Infant - 2nd
Lively boy - 3rd
Young man - 4th
Tired, middle-aged man - 5th
Grandfather - 6th
Cradle to the grave picture - 7th

You'll then get another Crest. I wrote this because I wasn't sure if you were indicating that you'd solved the puzzle or not.

As for the fourth Crest, this is probably where you have to use the Mansion Key you obtained. Examine it. If it's the Shield Key, then head up to the corridor where you met Richard. Enter the corridor behind him and then head up the stairs. Use the Sheild Key on the door. You will be attacked by a giant snake, called Yawn. Defeat him (watch out for poisoning!) and grab the crest from the hole at the back which he entered and exited out of.

what does "give me the peace of death, and i'll give you the joy of life..."

You have to click on the pictures stating with the youngest to the oldest then go to the very last picture at the end of the hall and turn it on