Answer
$24$ different ways
Work Step by Step
Lining people up involves permutation (because order matters) of 4 distinct objects taken 4 at a time.
Use the formula $P(n,r)=\dfrac{n!}{(n-r)!}$:
$P(4,4)
\\= \dfrac{4!}{(4-4)!}
\\= \frac{4\times3\times2\times1}{0!}
\\= \dfrac{24}{1}
\\= 24$
Thus, 4 people can line up in 24 different ways.
