Q. What is the correct way to write “four o’clock” as a book title?
A. Treat the contracting apostrophe in “o’clock” as if it were a hyphen and write Four O’Clock—capital O and C. The O is capitalized as the first word in a compound term, and the C because clock is a noun. If it helps, think of the title as short for “Four Of-the-Clock” (rather than “Four of the Clock”—though the latter is how the term would normally be spelled out).
See also CMOS 8.161, which explains Chicago’s rules for capitalizing hyphenated compounds in titles.