Pick the first guy 4 ways.

For each of those 4 ways, there are 3 ways left to pick the 2nd guy.
Thats 4*3=12 ways to pick the first 2 guys.

For each of those 12 ways, there 2 ways left to pick the 3rd guy.
Thats 4*3*2=24 ways to pick the first 3 guys.

Then there is just 1 way to pick the 4th.

Answer 4*3*2*1 = 4! = 24

Edwin