Write this out, let the number be x.
sqrt(2x) = x - 4 since 4 less means subtract
Square each side:
sqrt(2x)^2 = (x - 4)^2
2x = x^2 - 8x + 16
Subtract 2x from both sides
x^2 - 10x + 16 = 0
Using the quadratic calculator, we get two potential solutions
x = (2, 8)
Well, 2 does not work...