I have two things to say :
1) How many relay do you have to make this BCD counter?
2) Good luck! I tried to figure it myself. The first thing to know is how to maintain when an impulse arrive. To do this, you need one more relay on each section (see my figure). The seconde thing to do is the be able to make the relay switch AND maintain on impulse. The third thing is to make an adder with this!
The figure shows two relays with one as an auto-maintain relay. When the button is pushed, the first relay close and make the circuit close between Vcc and ground. As it is done, the second relay makes another road to ground. So, when the button is released, the second relay remain close.
(sorry for my bad english)