If the sub only has a single voice coil, then there is only one way to wire it. The key is all in the terminology...technically the load will still be 4 ohms. However, most amps that can handle a 2 ohm load in "normal" operation can only do a 4 ohm load in bridged mode. Usually if an amp is labeled as 1 ohm stable then it can do 2 ohms bridged, which would let you wire up two 4 ohms in parallel. However, much like computer parts, many amps that are 2 ohm stable can still do 2 ohms in bridged mode, it just isn't recommended by the manufacture, nor is it warrantied against. When I had my system with a 2 ohm stable amp, I had it bridged and ran two 8 ohm subs for a 4 ohm load in bridge mode. Hope this helps.
Smithers