How we cite our quotes: (Volume.Book.Chapter.Line)
Quote #4
She found after this, for the day or two, more amusement than she had ventured to count on in the fact, if it were not a mere fancy, of deceiving Susie. (1.5.4.5)
After deceiving Susan, Milly finds a sense of pleasure in realizing that she can do whatever she wants. She's spent the better part of the book so far under Susan's constant attention and observation, and is more than ready to spread her (dove) wings and fly on her own. It's just a shame that she only discovers this after she's learned about her illness.
Quote #5
"You can find out where they are. If I myself try," Mrs. Lowder explained, "I shall appear to treat them as if I supposed them deceiving me." (1.5.5.18)
Mrs. Lowder wants Milly to basically be her "spy" and to find out whether or not Merton has returned to London from New York. Maud suspects that Merton and Kate are meeting behind her back, but doesn't want to appear as if she's suspicious. She knows that if they find out she's conspiring against them, it'll end up bringing Kate and Merton even closer together.
Quote #6
But Milly's answer had prepared itself while Aunt Maud was on the stair; she had felt in a rush all the reasons that would make it the most dove-like; and she gave it, while she was about it, as earnest, as candid. "I don't think, dear lady, he's here." (1.5.6.23)
Once she knows about her illness, Milly starts becoming a lot more independent, and this independence is reflected largely in her willingness to deceive people. For example, she knows almost for a fact that Merton is back in London and meeting with Kate. But she decides to deceive Aunt Maud by telling her the opposite. We can tell that she resents Aunt Maud's attempt to make her into a puppet, and so she responds with deception.