Go to Best Buy and view the 4k demo tv's. The Sharp and Samsung demo's are impressive looking to me, the Sony less so. In particular much of the Sharp demo looks to actually have been shot with a 4k cinema camera.
These 4k TV's though, clearly are the best TV's ever made (other than perhaps the current or future OLED TV's, which I've not seen in person). Pixel detail aside, the color, the contrast...it's mind blowing.
But most of the content you would view at home, would be 1080p or less, and the 1080p-upsampled-to-4k, A-B test Sony has, doesn't look all that impressive to me. Obvious aliasing artifacts around small detailed objects like people skiing on the top of a snow covered mountain, shot from a helicopter. Of course I was about 2 feet in front of a 50 inch screen, when I saw this. But that's how close you have to be to a 4k tv, to be able to see all the detail!
I was happy for a long time, watching DVD, then 1080i broadcast, then some Blu-Ray's...on my 720p Panasonic front projector, whose screen is a Da-Lite high power (uses microscopic glass beads). The screen's width is 112 inches at 2.35:1 aspect. Still have the screen and surround sound, but sold the projector last year. Didn't feel like spending 2k to $3k on a new one just yet (especially given that I've spent more than that on camera gear this year!!)
That old projector could accept 1080p/24 via HDMI 1.3a, and Blu Rays looked astoundingly better than broadcast 1080i (or 720p) looked (even though I technically wasn't seeing all the pixels!). On my current (and small) 46 inch Samsung 1080p "smart tv", Blu Ray only looks a bit better to me than it did on the large screen via that projector that was only 1280x720. The oddest improvement is going from DVD to 720p or 1080i ("standard" HDTV broadcast). It absolutely kills DVD...
As for watching web videos, I don't do it much (other than on my new Asus 24 inch IPS desktop monitor, which I love...it's not perfect...but for the price it almost is). I watch Netflix on the Samsung occasionally, but the best signal I get from them is 720p. Not sure if it's my slow internet, or if that's just as high as they go. Either way, most of the time Netflix's picture quality appears a bit below that of satellite broadcast 1080i/720p, to my eyes. Sometimes it's very bad, because the feed, or the content, gets limited to as low as 320p!! I know because my tv can display the signal resolution...
As for your last comment, there is no such thing as a DVD that is 1080p. All DVD's are 480p at best. That's the standard. A DVD player can supposedly upsample and output a 1080p signal, but like the Sony comparison I mentioned above, it's obviously not always an actual improvement.