Jeez, my post above sounded a bit up-my-own-ar*e, I do apologise, it wasn't meant that way and I wish I'd edited it a bit better before posting.
Just to clarify, I'm not using the IR illuminator that came with the sight. There is a chap in Lynton/Lynmouth that sells IR torches through eBay under the Opticfire branch. I don't work for him nor am I affiliated, he's just handy to buy from because to me he's fairly local. If you do try him, I'm sure you'll find him helpful and knowledgeable. He also sells PARD kit. His Opticfire torches come with options of IR wavelength, focus, battery capacity and power levels. With my 4K, I am using a three power 850nm LED, 2 x 18650 batteries and the 67mm head unit. I find this is a good setup and as above, it will be lighting up eyes out to 250m. The three power option on the IR torch is a good option to choose because I like to use low power then ramp it up when need be. I also have a 760nm LED (I think it's 760nm) which enhances detail but at the cost of being more visible. I switch between the two and when I have used 760nm, it's not spooked Charlie enough to make them shy away.
One thing to be mindful of the ATN is that even with the rubber eyepiece in place, there is a lot of visible white light coming out the back of the unit, something to think about from a concealment perspective.
Please don't think I'm a ATN junky, I have a decent regular scope which I quite fancy adding a Pulsar 155 front mount IR unit onto, when the PPI money comes in
If I'm being critical, the ATN's daytime performance is not great and in bright sunlight it can be difficult to pick out detail even with the sunshade in place. I understand some users have fitted solid caps with an aperture, I've not tried that.
So, in my opinion, the ATN 4K suites my night-time shooting and I'd say it is worth considering if you shoot mainly at night and you've worked out what is the best illuminator for your quarry at distances your comfortable with. If you want a 50/50 day/night scope that works well at night but can also see a sparrow at 300m in bright sunlight, I'd say consider something else. So far I've had no issues with performance so I can't say how good or bad their customer service is.