From Förberedande kurs i matematik 1

\displaystyle \text{lg 1}0000\text{ } is that number which should stand in the grey box (??) in the equality

\displaystyle 10^{??}=10000

and because

\displaystyle 10^{4}=10000

we see that \displaystyle \text{lg 1}0000=\text{4}.