Can someone please explain to me the difference between single 4 ohm and dual 4 ohm?

 

Ohm's refer to the impedence of the speaker. There is Single Voice Coil, and Dual Voice Coil, each voice coil has an impedence. A SVC 4 ohm sub has an impedence of 4 ohms. Its equivolent would be a DVC 2 ohm sub, where there are 2 voice coils both rated at 2 ohms, together making a 4 ohm sub. There is also a DVC 4 ohm sub, where each coil is rated at 4 ohms thus making the sub an 8 ohm sub. This all has to do with the way you wire the subs to an amp.
Check out these links, they are very helpful:
http://www.jlaudio.com/tutorials/dvc/index.html
http://www.jlaudio.com/tutorials/wiring/index.html

I have two Diamond Audio dual 2 ohm subs wired series/parallel to run them at 2 ohms. The lower the impedence you run the subs [granted they're wired correctly], the harder the amp has to work, generally only run subs at 2 ohms or 1 ohm if the amp is made to handle it.

How's that for a first post.

 

Thanks Jib. Very informative Welcome to CL.

 

Thanks, I look forward to my stay.