I've hunted for 6 days using the S4 lockdown so far this fall. I have been happy with the S4 system but it isn't as convenient as a traditional Bino harness in my opinion. This is because you have to un strap you binos before you can look in them. I can live with the inconvenience because it protects my optics so well. I also like the way the harness straps don't interfere with my pack straps. All in all I would definitely recommend the S4.
Which size would you recommend for Nikon Monarch ATB 8x42.
I'll have to say I don't like my S4 lockdown. I bought a set before my trip to Montana after reading so many good reviews on them. They ride up the back of my neck and it drives me crazy. I've tried several different ways to adjust them, but get the same results.