I'll say it again - SPOILERS as I want to talk about the end of series 4. Don't read on if you don't want to know! And please please don't talk about series 5 as I haven't watched that yet 
OK. So, did Walt poison Brock? If so, when did he do it? And who took the ricin out of Jesse's fag packet? And if it wasn't Gus who did it, why did he seem all guilty as soon as Jesse told him the drs suspected poison??