If you have the W4, do you NEED another earphone? The W4 is a politer sweet than the SP or MX model, meaning that it has possibility to bring a bit more emotion or emphasis from certain midrange instruments. The j-phonics is slightly more sterile, but remains incredibly focused in the bass and midrange with just enough sparkle up top. It isn't as spacious as the W4 in the midrange when shuffling from percussion to strings, for example, but as an image, it presents them slightly more powerfully.
I think the W4 will probably appeal to audiophiles more and the j-phonics to the musicians/exercise crowd more. The j-phonics' fit is unparalleled and its customisation quite legendary. It isn't as thick in the mids as the UM3x or the W4, unless you listen to the MX, which has a smoothed out top end, so the midrange seems more forward.
Space-wise, it's sort of hard to say as the SP throws a good image, but in a different way to the W4, for instance. The W4 has a lot of space in the strings and percussion, but not as much in the bass as the j-phonics. Each have their strengths, there, and are hard to give a good 'winner' badge to because each display these strengths in different areas.