Conway's Tetratet

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:

\[4\rightarrow 4\rightarrow 4\rightarrow 4\]

in chained arrow notation. It is the smallest nth Conway Number to be larger than the nth Ackermann number.