"I won."
Yeah, Walt's descent into darkness is almost absolute. He had Saul's guard jack the ricin off Jesse to make it look like Gus poisoned Brock with it. But he hasn't lost all sense of morality. By using lily of the valley (of the shadow of death) he aimed to minimize harm by making it look like it was ricin, when it was in fact less likely to kill the kid. He slips up at the end when Jesse tells him Brock will live - "Oh thank God." Jesse doesn't clue in at the time - too relieved himself - but why should Walt care so much whether a kid he doesn't even know lives? Of course, it's because he's not a complete monster; he doesn't have the death of an innocent chemist and a child on his hands. But he's still bad enough to risk maybe killing the boy to save himself and his family.
It was a convoluted plan on his part, but another way to see it is as a last-ditch gambit. He didn't have a choice, and it worked. Jesse believed Walt had done it, and Walt managed to convince him Gus had done it, all without actually doing lasting harm to Brock (fortunately).
It leaves Walt's character development in a very interesting place. He's willing to go all the way to win. He's forced into ugly situations and it's a question of what would you do? Well I'd rather die than poison a child, and I would have probably kidnapped Gayle instead of shooting him - a less definite solution, but also easier on the conscience - but Walt is clearly prepared to go all out.
Poor Gus. His death scene made me want to cry. Partly because it was so cheesy. A total badass antagonist like that needed more than that anticlimactic finisher. It would've been better to show him lying against the wall in the clearing smoke, with the massive burns and chunks blown out of him, completely dazed, making to straighten his tie before slumping over. And a longer shot, please. He deserved better than a cartoon gory death. Gus, I salute thee.
That was the only low point of the season, similar to Mike's Mission Impossible montage at the end of the last one as both were out of place, even though Mike's was awesome.
Though the bugged glasses would've definitively explained Gus's suspicion with the car, you could also explain it with Jesse unwittingly tipping him off, as Walt surmises at the start of the episode. I think this part had a believable resolution.
I realised the glasses weren't tapped, and subsequently that Walt really had poisoned Brock with something other than ricin, when Gus actually went to see Tio. If the glasses were tapped, he'd have known about the plan. Gus played his chess moves too conventionally and that's how Walt blindsided him.
So where to from here? Both the cartel and Gus's operation are effectively collapsed, leaving a total meth void. (It seems Gus actually had minimal staff, probably because he was overly cautious, and Mike is the only one we know is still alive, assuming he survived getting shot up.) This leaves it free for Walt and Jesse to step back in and continue manufacturing with total monopoly. But I don't think that's going to happen, after everything they've been through. More likely, Season 5 will focus on the aftershock of everything they've done.
- The DEA now knows Hank was right about the laundry, seeing as it's burnt to the ground and left a whole lot of charred cooking equipment. There's also the high body count of both the cartel and Gus's lab.
- Mike is going to return to find his boss dead and the entire scheme burnt to cinders. Probably won't be happy.
- Jesse will probably eventually clue in to what Walt has done.
- Skyler knows her husband has killed at least three men.
- Jesse and Walt are both effectively broke, even after four seasons of fucking hard work.
So I think Season 5 will have the DEA, Jesse and Mike all closing in on Walt. I don't see it ending well for him, but you never know, he's played his cards expertly and almost ruthlessly so far. And they might just kill him off with cancer. They alluded to that in an earlier episode.
EDIT: Extra point. How did Saul's guard get the cigarettes back onto Jesse's person? Also, by what means did Walt poison Brock? Since Brock himself didn't know he'd been poisoned, Walt must have slipped it into his drink or something. At any rate, when he recovers, he'll probably be adamant that he didn't eat any strange white flower.