It's been a while (6 months!) since I posted challenge #4. My apologies. I'll try to be a bit more dependable in the future.
I'm not sure if I should brag or hide in a corner. Ticket to Ride: Europe is my own puzzle, and apparently it's a monster challenge:
Ticket to Ride: Europe  - 224 days, 6 solved - 0.03
That's right... Only 6 people have solved this puzzle in the 224 days that it has been available on CG. This is far-and-away the lowest solve rate of any CG puzzle: 0.03 solutions per day. Here are the published solutions as they stand right now:
Congratulations to you, JBM, Plopx, RoboStac, and magaiti. I salute you for your puzzle solving skills. Anyone else care to join their ranks? Give it a shot!