4" bass --for real? Does not exist. Check the specs. Which-ever has the lowest freq response wins. Kappas are aweful. I had them in 6.5". They were way too harsh.
There are several 2-5 1/4" size woofers/subwoofers on this site. I haven't personally used any of their products, but I've read lots of people on forums who have all with good results.
Gary, try J&M audio they make several different types of speakers for H-D fairings. Titan, makes an amplifier that mounts inside the fairing. Finally, in the new J&P cycles catalog, there is a subwoofer that you can mount inside the "tour pack" Happy hunting! BC