Keep looking for magazines. If the class calls for four magazines, it's a sure bet that some of the drills will require you to reload four times before refilling your magazines. You really don't want to be the guy slowing the rest of the class down because you're frantically stuffing mags while everyone else is standing there ready to shoot.
Yes, a good instructor will do his or her best to flex around your needs, but the equipment list is there for a reason. It is to make the class flow as efficiently as possible for everyone involved.
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