x4-16=0=>(x2-4)(x2+4)=0
x2-4=0=>x=2,-2
You are talking about the other equation
x2+4=0=>x2=-4=>x=√-4
These are special kind of numbers called complex numbers.You will read these in 11th or 12th class.In these √-1 is represented by i(iota).
So our x becomes=√4√-1=2i.
I have not taken the other value which will be -2i.