There are only specific channels that air 4K content and even then the broadcasters don't offer 4K content 24/7 for the simple reason of there is currently not a lot of 4K content available from those broadcasters. Most broadcasters are still only broadcasting in 720p or 1080p. Until more broadcasters make the move to 4K, content will be limited. One other important thing to remember is that any older content that was not recorded by the broadcasters in 4K, won't be viewable in 4K.
The first question would be - do you even have a 4K Optik set top box? If you have in fact had Optik TV for many years as you state, just because your TV can do 4K doesn't mean the Optik box can. You need one of the 4K boxes. If you do have a 4K Optik box and subscribe to the 4K channels, you also want to make sure you don't have your Optik box set to 720p or 1080p and that the HDMI cable you are using is capable of supporting 4K as well.
@DavidF when you say you get 720p where are you looking to determine that? If you have a 4K or even a full HD TV then you either have a connection or configuration problem. Even with the older HD Optik box you should be able to get 1080.
Providing details like your TV model, Optik box model and your connection method would go a long way towards getting the help you are asking for.