You can go for 3.3v but be sure to have good airflow on it. I mean, get a fan RIGHT over them. My source is OCZ Ryder, an official OCZ support guy that is sometimes on this forum. The VX4000 is rated 2-2-5-2 at 250 at 3.2v. Go for 3.3 just to be sure it's not a power supply issue. And yeah disable that CRC (2t), it is imperative with 4 sticks.
I have VX3200 (2 sticks) but will receive 2 sticks of VX4000 soon. I'll try 4 sticks for fun, but will probably go for VX4000 and sell those 3200. If you want, I could tell you the results when i get them.