Upgrade was definitely worth it here. I purchased a 4k UHD TV last Black Friday and already had the ATV4. My wife bought my the ATV 4k for Christmas and I have absolutely loved the upgrade. Movies I rent on iTunes look superb and really anything (Hulu, Netflix, HBO) look fantastic on the TV.
I got mine from the DTV deal too. I went through a few hoops to get it shipped to me in Canada but it was definitely worth it over the non-4k ATV.I have both. But, I got mine "free" with DirectTVNow for 3 months that I was going to buy anyhow.
Both are great.