An issue with Poison is that it reduces after dealing damage every turn, so unless you can apply a decent amount of it, it'll quickly fall off entirely. Four isn't very much, so you'll want to make sure that Haze is getting support from other Poison cards in your deck, but if you're building a deck with enough Sly support to care about the card at all there might not be much room for that. It's a bit of an awkward card only useful in a fairly narrow situation.
