I have three cork coasters with tsumego problems on them. The box says "Tsumego coaster" (詰碁コースター) and Nihon Ki-in (日本棋院), the coasters say 日本棋院 and the problem number. I'd like to know more about this series. Do the problems come from another source? Are they still in print? I only have problems 4, 5, 6. Here is problem 4.

Coaster No. 4