Futureproof is good, but it's not always the way to go.
You may be tempted to use DIRECTV's C61K 4K Genie Mini Client with a non-4K TV so that you won't need to replace it. I'll tell you, the C61K works almost as well with an HDTV 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.
Folks, I wouldn't do this. If you're going to go 4K, get the client and the TV at the same time. If you need to add a client today, add an HD Genie Mini Client instead and upgrade the system to 4K with a new client when you need it. The 4K Genie Mini is larger, uses more energy, and it's just not designed for HD use. It's just not the right way to go.