This follows directly from the double angle identity for sine,
sin(2u) = 2 sin(u) cos(u)
But supposing that's not known to you, you could use the angle sum identity:
sin(x + y) = sin(x) cos(y) + sin(y) cos(x)
Since 4u = 2u + 2u, we have
sin(4u) = sin(2u + 2u)
sin(4u) = sin(2u) cos(2u) + cos(2u) sin(2u)
sin(4u) = 2 sin(2u) cos(2u)
