I switched my cable box to get the 4K box since I have a Samsung QLED 4K tv. I still notice a low video quality even after switching. It looked like a 720p resolution when watching a baseball game tonight. I even replaced the HDMI cable with the one for my Xbox One X and there was no difference. Is this an issue that requires a technician to come fix?
It looks like 720p because that is the resolution used by the channel.
A 1080i channel will look worse the way the box handle conversion. Watching over the air is better for 1080i channels.
Broadcast content should look exactly the same as it did before, because it is almost all 720p with the local channels possibly 1080i if that is the native format for the station.
If it looks worse than before, try some different settings on the TV and box to see which does the upscaling better.
If you have Amazon Prime or a Premium Netflix subscription you can find some 4k HDR content to see if the box->tv is working at the 4k setting. Youtube also has 4k content but the XG1V4 does not display HDR with Youtube. If you say '4k' to the remote, you can scroll down to a few free movies (documentaries on Australia and turtles, etc.) that show the capabilities.
720p is as good as you ever see sports with the rare exceptions listed in the 4k faq. Some people complain about Comcast's compression but you shouldn't see that on your local broadcast channels. If yours doesn't look as good as it does in the xfinity store you might still need some adjustments.
When I first hooked up my xg1v4 I didn't realize my Sony TV needed to use only HDMI 2 or 3 for 4k HDR @60hz and also needed a change in settings to 'enhanced' mode. I'm still confused as to why the picture looked slightly blurry without that instead of not working at all, but I wasn't happy with it until making those changes. You also need an HDMI cable capable of carrying 18GHz, but most short ones should work. Can you verify that those 4k free movies work and show up in HDR on your TV? (The tv probably has some way to display the signal it is getting). If that looks good, then look at any motion or video enhancement settings on the TV that might affect sports action.