Let the present ages of two friends is x and y years
Hence x+y=20 or y=20-x
four years ago their ages will be x-4 and y -4
so,
(x-4) (y-4) = 48
(x-4) (20-x-4) = 48
(x-4) (16-x) = 48
(x-4) (x-16) = -48
(x-4) (x-16) + 48=0
x^2-20x+112=0
that
will give us two imaginary roots
But age should be in natural numbers
Hence, it is not possible.