An anapestic foot consists of two short syllables followed by a long syllable. A tetrameter consists of four feet. So, an anapestic tetrameter line consists of four anapestic feet, for a total of eight syllables.
Here are some examples of anapestic tetrameter lines:
* "The Assyrian came down like the wolf on the fold" (Lord Byron, "The Destruction of Sennacherib")
* "And the skies, like a dome of blue crystal, were bending" (Thomas Moore, "The Harp That Once Through Tara's Halls")
* "Then the little Hiawatha learned of every bird its language" (Henry Wadsworth Longfellow, "The Song of Hiawatha")