Indi, I haven't had my 4k very long, but so far I like it and haven't had any issues. Of course that could change with time. I like the rechargeable battery that is advertised to run for about 18 hours. I don't know just how long it will power the scope just yet, though. I charged the battery when I first got the scope and haven't had to recharge it since. I have turned it on and off numerous times over a few weeks, shot it some and it only shows one bar down on the battery scale. The gen 2 scope required an external battery for any real use, and that required a cable attached to a battery. I mounted a small battery on the left side picatinny of the X-Sight 2 and it would power the scope for about 4 hours with most features turned off. The 4k is self-contained and has no need for external battery or cables. The only cable necessary might be a remote switch for an IR illuminator.
Also, I like the rotating dial on the left side of the scope that allows for zooming. The scope seems to zoom much better without the lag that the second gen model has.
I like the eye relief of the 4k, which is an improvement over the gen 2 model.
Another particular thing I really like is that the 4k boots up so much quicker than the gen 2 model. The gen 2 would take several seconds to boot up once the power button was pushed, sometimes taking up to 8 seconds or more before getting a crosshair and being ready to shoot. The 4k will boot up in half that time. This is one area where the Photon excels, or at least the XT model does. It will boot up within one or two seconds.
Also, whiteout when shooting doesn't seem to be quite as bad with the 4k, though I admit I haven't shot it enough yet to make an absolute determination about that. It just seems to be a bit less as of now, and definitely less than the Photon.
The image through the scope does not seem to drag as much when the scope is panned around an area. The gen 2 scope seems to have a slight delayed action when scanning, causing the view and / or video to appear somewhat blurred or delayed.
I think the 4k has a slightly better image than the second gen scope and doesn't seem to pixilate quite as much when zooming, though it still does some.
The 4k accepts 30mm scope rings which will allow you to mount it slightly lower to the rifle bore, depending on the rifle and bolt lift, which results in a better trajectory. I purchased UTG high quick detach rings from Ebay for slightly over $20 and they work great and lock up tight. I don't know if they will return to zero or not since I haven't tested them yet, but I actually plan to keep it mounted on an AR upper, so it really won't matter.
As to a negative, the 4k is a bit picky about mounting on a bolt action rifle from what I have seen and read, due to its design. The eye relief, while better than the older model, is still not as long as I would like it to be. The scope tube isn't long enough to allow for positioning the scope more rearward on a bolt rifle. It seems better suited for an AR. The 4k does come with scope rings, two sets actually. One set has a funky looking offset ring that will allow the scope a more rearward position. For use on my AR, I much prefer the UTG rings.
Another negative is one that is common to many night vision scopes in general, particularly digital scopes, and that is field of view. The FOV is more narrow than I would like on both ATN models, including the Photon, which leads me to bring up another subject, which is magnification. For most reasonable shooting, say 150 yards and under, the 3-14x scope is the way to go. If you generally shoot distances over large open fields then the 5-20x would probably serve you better. I tried a 5-20x in the woods behind my house and it had way too much magnification. Magnification combined with small field of view doesn't work very well in those types of areas.