Answer: If a line has four feet, a poem is written in tetrameter.
Explanation: The terms "trimeter", "tetrameter", "pentameter" and "heptameter" are used to refer to the structure of a poem, more specifically to the number of feet that each line of a poem has. The term to talk about four feet is tetrameter, a word which contains the prefix "tetra-", whose meaning is "containing four". On the other hand, "trimeter" makes reference to three feet, "pentameter" refers to five feet and "heptameter" alludes to seven feet.
