It's hard to have a conversation because we all have different "before" points (what HDTV's we're using now) and different "after" points (what 4K HDTV's we're demo'ing in retail stores).
What I can tell you strictly from my own experience is that I currently own a 55" Sony XBR LED 1080p HD panel that is an absolutely stellar 100/100 for 1080p content and what I'd call a 95/100 for 720p content over Verizon FiOS which is generally thought to be the best producer of HD content from a resolution/quality standpoint.
When I go to Best Buy and see that same FiOS signal pumped through 4K panels from Samsung or LG and put on a 720p football broadcast on FOX or ESPN I see a significant amount of screen-door and motion blur, it's unacceptable to me.
So....if your current HDTV isn't as fabulous as mine and the 4K panel you're looking at has better upscaling than what you're used to then I can see how you might think the 4K panel is "just as good as the 1080p panel" for 720p native content. But, trust me, it isn't. Any 2016 model 1080p panel from a reputable maker's high-end line will produce a much better 720p experience. It's physics. There are simply millions fewer pixels that the TV has to fake to get 720p to fill up a 1080p display.
To the OP: Avoid 4K panels. Get the very best 1080p panel you can that has best-in-class upscaling and run with that for the next 5+ years. 4K content is at least that far away when it comes to NFL, NHL, MLB, NBA, NCAA, etc.
BJ