## Spin product of graphs

I don't understand this very well.

Is $$S_iS_jS_jS_k=4$$?
 No. You have sum. So Spins may take values +1 or -1. Because of that result of $$S_iS_jS_jS_k$$ is either +1 or -1. For example $$\sum_{S_1=-1,1}\sum_{S_2=-1,1}S_iS_{i+1}=-1 \cdot (-1)+(-1)\cdot 1+1\cdot(-1)+1-1 \cdot (-1)+(-1)\cdot 1+1\cdot(-1)=0$$ I don't know what that means in the graph case. If someone knows it would me nice to write down.