not many 4 ohm 12s out there. I was recommended the Faital 12PR300 (US Speakers has it) for a project, and based on a little WinISD modeling it looks very nice. And yes, it comes in a 4 ohm version, a bit pricey at US$180, though. Assuming you'd go DIY..........
California Bassist #24, TB Cellist #8, Honorary Georgia Bassist