4 ohm or 8

Gonna replace my bose subwoofer speakerm its a 1 ohm POS.
I have a Soundstream 400 watt 4 channel. The new fosgate 6 3/4
sub fits perfect but which one to get??

http://www.crutchfield.com/cgi-bin/S-3Yq7kcgVg36/ProdGroup.asp?c=10&g=85000&s=0

4ohm, you would use the 8 ohm if you were using two subs and needed to get to a 4 ohm load. You'll already be at 4ohm with one 4 ohm speaker.

HTH