Abaca Audio 4 Inch High-End Shielded Woofer 4 ohm: perfect for replacing/upgrading or building bookshelf, surround, sound bar speakers, or other diy speaker projects.
Amazon.com: Rockville 1500w 15" Raw DJ Subwoofer 4 Ohm Sub Woofer 70OZ Magnet, 15 inch (RVW1500P4) : Electronics
BOSS Audio Chaos Exxtreme 12 Inch 1200W DVC 4 Ohm Car Audio Subwoofer (4 Pack) - Walmart.com
JL Audio 12W0v3-4 W0v3 Series 12" 4-ohm subwoofer at Crutchfield