From Förberedande kurs i matematik 1

\displaystyle \mathop{\text{lg}} 10000 is that number which should stand in the coloured box in the equality

\displaystyle 10^{\bbox[#FFEEAA;,1.5pt]{\phantom{\scriptstyle ??}}} = 10000

and because

\displaystyle 10^4 = 10000

we see that \displaystyle \mathop{\text{lg}} 10000 = 4\,.