Sugar cane can generate 4 blocks high

It's a thing (bug? not sure) in the world generation

"Sugar cane may grow to a maximum height of three blocks, but is occasionally found in new world generation up to seven blocks tall."
-http://www.minecraftwiki.net/wiki/Sugar_Cane

These kind of minor oddities make exploration more fun and don't hurt anything.

"It's not a bug. If two sugar canes try to generate in the same place, they stack on top of each other."