This one’s easy, but you won’t like the answer.
Although the Genie 2 can support up to 7 receivers or smart TVs, only two of those can be 4K. You can use the C61K 4K Genie Mini (which is what I would recommend) or you can use a DIRECTV 4K-Ready smart TV.
I personally prefer the client box to a smart TV just because TV manufacturers have been slow to update their TVs when there is a problem, while DIRECTV has been much quicker, fixing problems in days that take TV makers months to roll out.
Technically wireless clients can be used, except there aren’t any. If you have a 4K TV with built-in Wi-Fi, it can’t be used for DIRECTV 4K. You must wire the TV using coaxial cable and a DECA adapter or it won’t work. The fact that wireless clients are supported tells you that one day there will be one, but no one knows when that will be.
If you want more 4K than that, unfortunately DIRECTV isn’t really ready to supply it. DISH’s products can supply pretty much every TV you have with 4K, but of course they have no 4K channels so the only 4K content you would get would be from Netflix and you could get that with a smart TV anyway.
DIRECTV is expected to add additional 4K channels in 2018, as soon as there are some to add, and as demand for 4K continues to grow, it’s expected that you’ll see new hardware that will make that work. There is some speculation in the DIRECTV community that the Genie 2 does support MoCA 2.0 which would be very capable of supplying 7 TVs with 4k (note that DISH’s Hopper 3 uses MoCA 2.0 to supply 4K to its clients). The theory goes that if there were MoCA 2.0 clients being used and nothing else, perhaps you could see more 4K client boxes supported by Genie 2. Personally I can’t confirm that, and I think we will all have to wait and see.