Are we talking the original fixed mark 4 or the newer variables?
I have both (NXS 5.5-22x50 & Mark 4 10x M1). Aside from magnification, I prefer my mark 4. The optics are clearer, it is just as durable and bomb proof, adjustments are solid, accurate, positive, reliable and repeatable. It is lighter, more compact. It offers the same amount of light gathering as my NXS even with 10mm less objective. This is due to it being a fixed power. Fixed power optics have much better clarity and light transmission than variables of equal size/power. Hence the reason it is as bright and much clearer. My 10x seems like I am looking through a 13-15x NXS due to its clarity and brightness.
If I had to buy a new scope tomorrow, it would be a Mark 4 16x M1. If it HAD to be a variable power, I would get another NXS over the second generation Mark 4.
All that said, I love my NXS and would recommend them as well. I like having the zero stop (even though it isnt really needed) and the illuminated reticle (even though I never use it and have never used it). I really dont care for the NXS glass but it is sufficient.
I dont really like carrying the extra bulk and weight. I use this scope as a sit in one spot and glass, glass, glass untill I find something to shoot scope.