If you're using a dual-plane (180 degree) manifold, then a 4-hole spacer may help the fuel/air mixture make the turn into the manifold runners a little better, rather than smashing into the floor of the manifold as it exits the carb bores and creating turbulence. You may see this improvement on the dyno, but I doubt that you'll feel it in the seat of your pants. You will however, add volume to the manifold with a 4-hole spacer, or any spacer for that matter. Generally speaking, the more volume in the manifold, the more power the motor will make up higher and the less it will make down low.
With a 180 manifold and an open spacer, you will effectively change your manifold from a 180 to a 360 degree design and kill off bottom end torque because now, each and every cylinder "sees" the entire manifold volume rather than just half of it like with a segregated 4-hole spacer, so low-rpm cylinder filling will suffer along with low-rpm torque. It's a trade-off. You'll make more power on the high end (assuming you have enough cam to make power up high), less on the low end.
My best advice: Leave it alone and save your money for something more important, like a set of gears or converter.