Chapter 2 - Section 2-1 - Sample Spaces and Events - Exercises - Page 26: 2-2

The sample space is: $S=\left\lbrace{eeee,eeen,eene,enee,neee,nnee,nene,neen, eenn,enen,enne,ennn,nenn,nnen,nnne,nnnn}\right\rbrace$

Let $e$ denote the in-error transmitted bits, and $n$ the not-in-error transmitted bits. Given each of four bits may be classified as $e$ or $n$, we'll have $2\times2\times2\times2=16$ elements in the sample space.