expl3:

\documentclass{minimal}

\usepackage{xparse,expl3}

\ExplSyntaxOn 

\def \expconst { \fp_eval:n { exp ( 1 ) } }
\NewDocumentCommand \mylog { m }
{
    \fp_eval:n { round ( ln ( #1 ) / ln ( 10 ) , 4 ) }
}
\ExplSyntaxOff 

%%% document
\mylog{2}
\mylog{3}
\mylog{\expconst}