Photographed and 3D modeled from a replica of a Netsuke. At first glance I felt that this was showing a kind of famine type message. The rats are all stealing the grain leaving none for others and there are four of them shown (the number four can be symbolic for death in Japanese culture).
If you rotate the object around you will see the fifth rat gleefully buried in the grain sack. For me this then becomes more of a symbol of prosperity and abundance. The happy rats have way more than they will ever be able to eat.