Haven't heard any reports of spring sagging, and mine aren't as of yet, but with that said I wouldn't recommend the 4" lift from zone if you do any wheeling. If you are going to stay on the street than anything would probably suit you fine, but if you wheel at all I would stay away from it, and here is why:
About a year ago I put the 4" lift from Zone on my 98 TJ. Over the last year it's done fine, but I've spent that time slowly adding what the kit didn't come with so that my Jeep would finally be more complete. That includes Front/Rear adjustable control arms (upper & lower) so that I could add an SYE, front/rear adjustable trac bars, etc... Those little things make a big difference.
The bad thing about the 4" lift from Zone are the fixed lower control arms. I recently got a "nused" '04 TJ and moved everything from my old Jeep onto it. When I removed the fixed arms from Zone, the bushings in them were completely shot to the point where I wouldn't have been able to reuse them had I needed to. The mounts were also pretty bent up, and required a bit of time with a hammer to get them in shape again. Had I left them on for another year the mounts may not have been there anymore. These fixed arms are probably the worst part of the lift; otherwise it isn't bad, it's just missing most of the nice features that really make the lift worth while.
On my "nused" TJ, the springs, shocks, and rear extended swaybar links are the only things left from Zone that I reused. And, the springs and shocks will soon be replaced as well to help with the ride quality. The new control arms I got improved things drastically, but the springs and shocks from Zone are a bit stiff for my taste. If you had 500lbs of armor it might not be bad, but my Jeep is pretty light.
Like I said previously, if you aren't planning on wheeling much, the Zone lift might work well for you. But in my experience, if you plan on wheeling anything challenging you'll want to look elsewhere. Don't waste money like I did.