Could be the spray. But it could also be loose pollen – have you checked for any male fowers in and around the plants? It only takes one male pollen sac to pollinate a large number of female flowers. If it was the spray, I would be inclined to think that most of the pistils would be affected, whereas a small pollen leak might only turn a few pistils brown. Some earlier finishing strains will also start to show brown pistils at around Week 4, so it could be entirely natural.
Your plants look healthy so you shouldn't lose your crop, but check again in a week or two to ensure no seeds have formed. If you find any male pollen sacs, remove them immediately and keep an eye out for more during the rest of the grow. Spray/mist the whole crop with plain water, as that will activate any remaining pollen so it can't pollinate any more female flowers.