How we cite our quotes: Citations follow this format: (Paragraph)
Quote #4
A bronze group stood upon the landing, hidden from me by the corner of the wall, but its shadow fell with marvelous distinctness upon the white paneling, and gave me the impression of someone crouching to waylay me. I stood rigid for half a minute perhaps. (29)
It sounds like the narrator momentarily thinks that there really is someone crouching in the hallway. This is the most obvious case of the narrator "thinking he sees something" in the story. It also indicates how active his imagination is, and how fragile his nerves might be.
Quote #5
I resolved to make a systematic examination of the place at once, and dispel the fanciful suggestions of its obscurity before they obtained a hold upon me. (32)
The narrator wants to make sure that the "suggestions" made by the dark ("obscurity") don’t get his imagination working actively. It is the dark specifically that he blames for stimulating his imagination. He blames the darkness this because it literally conceals things from sight (as it did with the Ganymede statue), and because it’s ominous and murky. That’s why he resolves to remove the element of mystery and murkiness in the room by removing the dark, first by making a survey of it with his candle.
Quote #6
My precise examination had done me good, but I still found the remoter darkness of the place, and its perfect stillness, too stimulating for the imagination. (33)
Another affirmation of the narrator. Apparently knowing what’s "really" in the room isn’t enough to prevent the narrator’s imagination from conjuring up all kinds of things. There’s still too much darkness in the room. He’s got to work on that.