What board do you have?
Your amp may be big enough to just daisy chain the kustom monitors. 2 monitors on each side of the amp so its running at 4ohms instead of 8ohms. You will get more power out of the amp that way anyways. Then you could just buy the 2 more monitors. Have 1 of your aux sends control the 2 monitors at the front, and then have the other aux send control the drummer's monitor. You could even just use the monitor you already have. Run that monitor on channel B of your power amp for the drummer through Aux 2, and then daisy chain the Kustom's from Channel A of your power camp through Aux 1. This way you don't have to spend any money, except for maybe a speaker cable. Even if daisy chaining your Kustom's and running the amp at 4ohms provides more power then the speakers are rated I think you will be ok. I have used Kustom monitors and I always give them double the power. What amp are you using? Hope this helps...