Yes, it works, but it nags you.
DIRECTV’s C61K 4K Genie Mini Client will work with a non-4K TV, almost as well as it works with a 4K TV. I have personally confirmed this. However, there are some issues that may be long-term or may simply be due to the fairly early build level of the software that runs it.
When starting up, I got a message about 50% of the time that this was a non-4K TV and that 4K wasn’t supported. Well, duh. But at any rate I had to choose “Try again” from a very sparse menu in order to get to live TV.
Obviously, I was not able to select 2160p as a resolution and 4K content wouldn’t play due to HDCP (content protection) issues. So your hopes of buying a 4K pay-per-view and having it display on other TVs in non-4K resolution are basically dashed. It does not downsample 4K programming; it just ignores it.
Personally I recommend people not doing this long-term, as it’s just not worth the nag screens. If you’re close to a 4K TV purchase, I could see it but otherwise there’s no reason for it.