I'm pretty sure it was on ff.net, although there's always the possibility that it was posted to lj as well. In the fic, when Cas breaks the great wall of Sam, Sam just remembers everything from his time in the cage and gets constant, violent flashbacks. Gabriel comes and turns Sam into a four year old (only physically, mentally he's still his real age) so that he's easier for Dean to manage during the hell flashbacks (they're really intense, etc.). Also, I'm pretty sure that Samuel Campbell comes at some point and kidnaps Sam to torture him for information on Mary or purgatory or bringing Mary back from the dead or something along those lines. Oh, and I'm pretty sure Dean and Sam were at Bobby's throughout the fic, and I remember a specific scene where Sam wanted coffee but Dean gave him chocolate milk instead since Sam's four year old body couldn't handle coffee or something. If anyone can link me this fic (or a similar one?) that would be lovely!