Without knowing what Juan's exact steps were, it's hard to say what he did wrong. The least you could say is that his solution is simply not correct.
4 sin²(θ) - 1 = 0
==> sin²(θ) = 1/4
==> sin(θ) = ±1/√2
==> θ = π/4, 3π/4, 5π/4, 7π/4
