Step 1: Read the blank as the subject of a well known saying about the danger of insufficient learning. Step 2: Test each option: "knowledge" (options 1-3) is a widespread misquote in everyday speech, but the poet Alexander Pope actually wrote "learning," not "knowledge." Step 3: Only option 4, "A little learning," matches the authentic wording of the quotation. \[\boxed{\text{A little learning is a dangerous thing}}\]