We pick 4 of them to evaluate ONE arbitrage strategy (fwd rate too high) and the other 4 to evaluate another arbitrate opportunity (fwd rate too low)
Across asset classes, most pricing theories assume 0 transaction cost and 0 bid/ask spread. In this case, the bid/ask spread is often the showstopper for the arbitrageur. Similar challenge exists for many option arbitrageurs.
I think [[complete guide to capital markets]] has one page illustrating this arbitrage.