Both are bottle episodes? I don't know, I've given up on trying to convince foolish people that Fly is a great episode, I can't even imagine having to defend 4 Days Out.
"A robot?"
There's so many great lines in 4 days out. That one's one of the best, but "How are you even alive?", "Ahhh.. wire!", and the entire scene where they calculate how much meth they made are all time favorites for me.
That's the thing, I can see why people would be down on Fly. All things considered, not much really happens plot-wise aside from Walt telling Jesse that he suspects that he's skimming. Still, I could probably watch an entire series about just watching Walt and Jesse stuck in a room together. They're both just so well written and acted, I was riveted throughout the entire episode.
On top of the awesomeness of that, though, 4 Days Out is pivotal, plot-wise. They make the huge amount of meth that they end up eventually meeting Gus to sell, and there's that absolutely great reveal at the end that Walt's cancer is in remission.