Conway's Tetratet, or Conway's four-four-four-four, is a number mentioned by John H. Conway in the Book of Numbers. It is the largest explicitly defined finite number in the entire book. It is also the 4th Conway number. The number is defined as: \[4ightarrow 4ightarrow 4ightarrow 4\] in chained arrow notation. It is the smallest nth Conway number to be larger than the nth Ackermann number. Using Bird's Proof, it can be shown that \(\{4,5,3,2\} \geq 4 ightarrow 4 ightarrow 4 ightarrow 4\) in BEAF. Conway's Tetratet is the output of the CG function, by putting the input with 4, CG(4).