How can the outermost electrons in silicon be 2s and 2p electrons?
The book means to say that there are 2 electrons in the  $\mathrm{3s}$ subshell and and 2 electrons in the  $\mathrm{3p}$ subshell. You can see this in the electron configuration as 


  $\mathrm{...3s^2 3p^2}$


and that the maximum configuration has 2 electrons in the  $\mathrm{3s}$ subshell and 6 electrons in the  $\mathrm{3p}$ subshell which would be 


  $\mathrm{...3s^2 3p^6}$