You can only wire that sub as either an 8 ohm or 2 ohm configuration. If you run only one coil you can ruin the sub. Best bet is to get another sub and then you can series/parallel the subs to get your four ohm load.
If a second sub is outta the question, then your only alternative is to sell the one you got and go with a single coil 4 ohm sub. Are you trying to use a 2 channel amp? If its a mono, most well known brands can go as low as a 2 ohm load or lower.
Thanks for the quick replies! The amp is a mono rated for 2 or 4 ohms, the sub is wired for 2 ohms now, but I thought it might sound better with a little less power. The bass over powers the rest of the sound, maybe I just need to tune the amp better.