I recently upgraded from Monitor 0 to the Omegas. You should take my comments as somewhat preliminary, as I'm still in the process of breaking in the cables, and one of the runs is REAL long (40 ft.--ouch!). They have been getting better and better as they've broken in. Also, the way cables sound in one system may not necessarily be how they sound in another, so take my thoughts with that grain of salt. Sometimes, if the source material or equipment has limitations, you may not prefer a more revealing cable, as it can magnify those limitations.
Disclaimers aside, the thing I always liked about the Monitor series was that it had a very good tonal richness, enough so that it would make other top quality cables like Nordost (not the Valhalla--that one closed the gap) sound thin and bleached out in comparison, while still letting through a ton of detail. However, it could sound a little slow, seemed to roll the highs a bit (until I got the upgrade it was tough to say that for sure, as my equipment does that too), and had a slight mid- to upper- bass thickness, for lack of a better word, that tended to impart a dark character to the midrange. Not at all unpleasant, I should add, particularly with classical music. The Omega is more neutral; the bass region in particular seems faster and more extended in the deep bass, although because it doesn't have that slight bump in the mid-bass it may not "seem" as extended or powerful in the bass. By cleaning up the bass, the midrange is better as well, with no congestion or slowness, very open. The highs are far more extended, without being bright or bleached out, much more "air" up there, even noticable with my Jadis equipment, which is a bit rolled off up there. And the noise floor seems substantially lower, as I'm getting much more ambient information from all my sources. I would say that the harmonic richness that I liked about the Monitor series is still there, although the Omega is not as "dark" sounding as the Monitor 0; it has a nice naturalness to it. All in all, it has taken some getting used to, but the verdict is that I'm glad I made the upgrade; my preference would put it a little above the Valhalla, which still sounds a little thin harmonically (really, only in comparison) in my system (but, I might add, works very well together with the Omega); the Monitor 0 would have been about the level of the Valhalla, just a different tonal balance and "flavor" to it. I will note that the Omega is VERY bulky, about 1 1/2 inches in diameter; it does, tend to stay the way you bend it, but it's a good deal tougher to fit in tight places than most cables. And it is terribly expensive, although the upgrade program is helpful if you've already got NBS cables. Hope this is of some help; I'm not sure how the Monitor 4 sounds in comparison to the 0, so maybe others can help you there.
Disclaimers aside, the thing I always liked about the Monitor series was that it had a very good tonal richness, enough so that it would make other top quality cables like Nordost (not the Valhalla--that one closed the gap) sound thin and bleached out in comparison, while still letting through a ton of detail. However, it could sound a little slow, seemed to roll the highs a bit (until I got the upgrade it was tough to say that for sure, as my equipment does that too), and had a slight mid- to upper- bass thickness, for lack of a better word, that tended to impart a dark character to the midrange. Not at all unpleasant, I should add, particularly with classical music. The Omega is more neutral; the bass region in particular seems faster and more extended in the deep bass, although because it doesn't have that slight bump in the mid-bass it may not "seem" as extended or powerful in the bass. By cleaning up the bass, the midrange is better as well, with no congestion or slowness, very open. The highs are far more extended, without being bright or bleached out, much more "air" up there, even noticable with my Jadis equipment, which is a bit rolled off up there. And the noise floor seems substantially lower, as I'm getting much more ambient information from all my sources. I would say that the harmonic richness that I liked about the Monitor series is still there, although the Omega is not as "dark" sounding as the Monitor 0; it has a nice naturalness to it. All in all, it has taken some getting used to, but the verdict is that I'm glad I made the upgrade; my preference would put it a little above the Valhalla, which still sounds a little thin harmonically (really, only in comparison) in my system (but, I might add, works very well together with the Omega); the Monitor 0 would have been about the level of the Valhalla, just a different tonal balance and "flavor" to it. I will note that the Omega is VERY bulky, about 1 1/2 inches in diameter; it does, tend to stay the way you bend it, but it's a good deal tougher to fit in tight places than most cables. And it is terribly expensive, although the upgrade program is helpful if you've already got NBS cables. Hope this is of some help; I'm not sure how the Monitor 4 sounds in comparison to the 0, so maybe others can help you there.