Let $x$ indicate the $x$ -th letter of the alphabet.
The number of letters before $x$ is $x-1$ and the number of letters after $x$ is $26-x$ since there are 26 letters in the alphabet.
Since we want the letters before to be four times the letters after, we demand that:
$x-1=4\cdot (26-x)\Rightarrow x-1=104-4x\Rightarrow 5x=105\Rightarrow x=21$
Therefore the letter we want is the 21-th letter which is $\boxed{U}$