SJ - Check out Patriot Performance Materials.
www.patriotmaterials.com They have a proprietary plate that is better than anything else I've seem. I would recommend discussing it with them personally as I'm not an expert. The price is about $1000 in Level III and $1200 in Level IV. Weight is < 7lbs ea and the test samples I saw had all passed multiple impacts (Level IV test protocols) w/i 3" of each other and w/i 1" of the edge. I was very impressed. FWIW - Peregrino