Hey guys, shoudl be really easy for you, but I somehow cannot figure it out.

Im looking for an explicit equation of the sequence $\displaystyle a_n$ with

$\displaystyle a_0 = 1$ (this would be good, but is not needed necessarily)

$\displaystyle a_1 = 4$

$\displaystyle a_2 = 24$

$\displaystyle a_3 = 192$

$\displaystyle a_4 = 1920$

You see, we multiply by 4, then by 6, then by 8, then by 10 etc., anyone can tell em an explicit solution?