That's the problem. You don't know what you have, and I can't help you without that knowledge. However, you probably have a 500Twin and a legacy Dual. If so, you will either need to change the 500Twin for 2 more legacy duals and get an SW64. Run 2 lines from 3 of the locations into the SW64, and connect ports 2 & 3 to SW21s to tie in that 4th location. Port 1 needs power insertion, and will kill an SW21. Complicated? Yup. Or, get a DP Twin, 2 DP Duals, and either a DP34 and 2 DP21s or a DP+44. Fairly expensive either way....