Busy with challenge #3, which seems a lot harder than challenge #4.. The amount of username combinations are larger, and there is no specification as to if they only mean "submitters" or also commentators. Also there are no username lists available for reddit so you have to parse it from reddit manually... Again, the rewards seem a bit skewed from the supplied text file challenge #3 got.

For example, did the OP consider that reddit posts can dissapear? Therre is a max amount of pages you can go through at /r/Bitcoin/ Or did they mean from the top posts? Again, hard puzzle.