There is only one number that satisfies x=4x, meaning the a number quadrupled is the same number.
That number is 0.
I guess if you want to be technical, then the square root of any negative number would also work. The square root of a negative number doesn't exist, it is what is called an "imaginary number". So in a way an imaginary number would be the same as four times an imaginary number.
If you want to get deep, then there's another case that works. Infinity. Which is a greater value, one infinity or four infinities? They're the same thing because they both never end.
