Walt needs Jesse on his side to get to Gus
Context: If you recall, Jesse is the one who tells Walt that Gus seems to have a big hatred for Hector "Tio" Salamanca, then Walt figured out how to plant the bomb on Tio, Gus died... etc etc
So Walt forms a scheme to get Jesse to distrust Gus. Walt tells Saul to somehow extract the cigarette from Jesse. Thus Saul has Huell lift the ricin cigarette from Jesse's pocket (most likely by simply trading out the packs, dummy pack for the real one with the ricin). Context: Originally Jesse was somewhat distrustful of Gus, Walt had hatched an earlier plan to have Jesse kill Gus with the ricin. But Jesse was befriended by Gus, and he eventually came to like the guy. Also, if you remember, Saul is frantic to get Jesse to his office, calling him over and over again. It was to get him in the office to get the cigarette off him.
Jesse now has a dummy pack of cigarettes. With this in mind, Walt now makes the moves to make Jesse distrust Gus. Walt takes his "Lilly of the Valley" extract and gives it to Jesse's girlfriend's young son Brock.
Context: Vince Gilligan, the show's creator, has stated several times that the writers have imagined Walt's delivery system as perhaps a doctored juice box or something of the like. Sneaking into Brock's school to place it in his lunch or even hand it to him would've been fairly rudimentary for a teacher. "Lilly of the Valley" gives pneumonia-like symptoms that appear very severe (the same symptoms that ricin gives when killing someone). So Jesse thought that Brock was poisoned by the ricin. Jesse frantically searches in his cigarette pack only to find, ah! It's not there! (Huell took it!) Jesse bursts into Walt's home, gun in hand demanding Walt to admit that he poisoned Brock with the now missing ricin. Important to note: Jesse says that Huell must have took it when he went to meet Saul. Jesse is no idiot, he was 100% right on his instincts. Walt claims ignorance, saying he has no reason to do so and he has no idea what Jesse is talking about (lying obviously). Through Walt's machinations, he convinces Jesse that it must have been Gus, who has hurt children before (Andrea's brother who shot Combo was killed by some of Gus' lower order thugs). Jesse now doles out the details of Gus' hatred for Hector "Tio" Salamanca leading to Gus' eventual death via Walt's admittedly ingenious scheme. At the end of season 4, the doctors at the hospital tell Jesse that Brock was not poisoned with ricin, but had consumed "Lilly of the Valley" berries in some shape or fashion. Jesse, taken aback, rationalizes with Walt that even though Gus didn't do it, he "had to go," although he is clearly still rattled.
Walt and Jesse go on a hunt for the missing ricin cigarette (although Walt knows exactly where it's at, and we're even shown Saul throwing the ricin cigarette back to Walt in a plastic bag, making a crack about Huell's "nimble sausage fingers"). Walt is just making a facade to make Jesse think the cigarette was simply misplaced. They "find" the ricin cigarette in Jesse's electronic vacuum (although it was Walt who placed it there).
Jesse breaks down in tears, realizing he almost killed Walt over this (as aforementioned when Jesse confronted him in season 4, saying he was the one who took the cigarette and poisoned Brock). Although in reality, the bastard did deserve it.
This episode! Jesse is ready to move on with his life, move to Alaska, and just leave ABQ. Saul tells Jesse he can't bring pot to the meet with his "guy." The guy won't be inclined to help a druggie disappear (sensible). Jesse defiantly and silently refuses to give up his stash. Saul leaves the room to get "money bags" and while he is out there he tells Huell to pinch Jesse's stash off him (rewatch the scene, you can actually see him snatch it from Jesse!).
Jesse is waiting at the stop, he searches his pockets, at first just simply realizing the pot isn't there. But he looks at his pack of cigarettes and realizes, holy ****, Huell took my pot just now... and they took the ricin just as I had originally thought. Walt has been **** me ever since.