Apparently lower quality signals look terrible on these TVs. Older movies for example. And I assume that would mean lower quality online content as well.
You may want to verify that you have a good connection. If your TV device, (digital box/cable), is receiving a strong enough signal for TV to show high quality for example. And/or if your internet connection is good enough to provide you with a good signal/bandwidth.
Are you in a remote area?
What is your proximity to your Wireless router or Access Point, or do you have a direct connection?
What is your upload/download speed as provided by your ISP based on your internet package?
It may not be the TV, or it may be. I'd try a BluRay movie to see just how good the quality can be, and/or you can search 4K content on Youtube to test there.i.e: https://www.youtube.com/results?search_query=4k+video+ultra+hd
Testing the 4K on the internet might give you some idea of how good your connection is, while checking a high quality movie on BluRay may give you some idea what sort of quality standard you should be looking at for regular and HD TV.
I just purchased a samsung 4k tv Model # UN55KU6290FXZA. I know I can't get too many 4k signals as of yet but I would have assumed that it would give me at the very least 720p picture. The picture through my dvr is absolutely awful and even worse when i try to stream through a streaming service. Can anybody tell me the problem. Everything is connected with HDMI cables. What could be wrong? Thanks in advance.