“Come back in an hour, Ferdie.” Then in a grave murmur: “His name is Ferdie.”
Besides the obvious that she wants Ferdie -- her driver -- to come back a hour later to pick her up and take her back home, this line is possibly here to further show what kind of person Daisy is -- as mention earlier in the book. I personally think think this is the reason for it.
To me it shows that by knowin her own Driver's name that she hasn't quite yet gotten so high class that she basically looks down or shun against people who are lower class to her because she feels she better or inferior to them than. Then again, she said it in a way that she didn't really want Nick to hear it, -- in a grave murmurish type of way -- which could be inferred that she feels she above of knowin such knowledge or doesn't care for knowin of such knowledge although she does or doesn't want others to know that she knows of such knowledge although she does. That it almost pains her to even know or even care for such information.
That isn't at all that rare in my opinion. Most people who have maids, butlers, etc, probably really don't care to even know their own employees name yet along their personal life. Daisy could feel that she should be of the same way if she wants to be friends or fit in with such individuals.
To help improve the meaning of these lyrics, visit “The Great Gatsby (Chapter V)” by F. Scott Fitzgerald and leave a comment on the lyrics box