There is no evidence for Walt poisoning Brock
There is no evidence for Walt poisoning Brock (as of Season 4). Just because Walt has that plant doesn't mean he poisoned Brock. I went back and watched the last few episodes of season 4. Walt never had the opportunity. He shows up at Jesse's house where Brock is playing a video game. Walt is not carrying the flower pot in the scene. He immediately gets into a fight with Jesse without even coming inside. Then he gets tasered by Tyrus and taken out to the desert. Brock doesn't live with Jesse so Walt has no way to figure out who Brock is or where he lives, so Walt never had the chance to poison him, even if you believe Walt is capable of doing that or wanted to.
Walt having that plant is a coincidence, just like how he met Donald before Jane's death, or like how Beneke dies while Huell is in his house. The show is full of weird coincidences.