Spacers
A spacer is used as a tuning aid. As a rule of thumb a 4-hole design spacer will increase the air velocity through the intake tract, which will generally give you better, throttle response, torque, and acceleration. An open spacer will decrease the low speed air velocity, but increase your higher RPM air speed. This will generally move your power band up higher. A 4-hole spacer can make up for having something too big in your combination (i.e. heads, intake, carb, etc.) while an open spacer can make up for something being to small.
We normally recommend building an engine without a spacer, and then using one if need be based upon your desired results. If you need more bottom end torque, try a 4-hole spacer, if you want more top end try an open spacer. Generally the thicker the spacer the more of an effect it will have.