How will the picture quality be with my cable feed. I have heard that the Spectrum resolution is not fine enough for 4K televisions.

Broadcast television is currently transmitted in 720p and 1080i resolutions or 1280x720 and 1920x1080 pixel dimensions. For comparison, this would be the equivalent of either 0.9 or 2.1 mega-pixel (approximately) still photos playing in sequence to create motion. 4K UHD TVs have a screen resolution of 3840x2160 pixels, regardless of TV size (approximately 8.3 mega-pixels), which means 720p and 1080i TV broadcasts have to be "upscaled" (or enlarged) to fill the screen of a 4K TV. This means that all cable company broadcasts from any service provider and free over-the-air broadcasts won't look their best on a 4K TV since they all have to be enlarged. High-bandwidth transmission options like fiber and satellite can sometimes look better than traditional cable transmissions since they often don't compress their TV signals as much (resulting in fewer compression artifacts like pixelization, banding, color loss and noise) but they still have to be upscaled to show on a 4K TV. For true 4K viewing, UHD Blu-ray and 4K streaming from services like Netflix and Roku are currently the only options. Your 4K home movies can also be shown on your 4K TV. Broadcast television standards are slow to change because of the complexity of issues involving industry-wide standards and FAA broadcast regulations. So don't expect 4K cable or local TV any time soon.

Other factors like viewing distance, TV size, the quality of the TV's upscaling hardware and software and personal preference play a role in how your cable broadcasts will look on a 4K TV. The smaller the screen at a given viewing distance, the less you'll notice upscaling artifacts. Likewise, the further the viewing distance from a given screen size, the less you'll notice the issues. I have Frontier FiOS (formerly Verizon FiOS in my area) and I've noticed it offers a slight but noticeable improved picture quality over Spectrum (formerly Brighthouse in my area) but your experience may vary.

Just a final note, even though we call 3840x2160 resolution TVs "4K" (meaning 4 thousand pixels across), it is obviously just short of true 4K resolution. 3.8K doesn't sound as catchy, though. There actually is a true 4K resolution that is 4096x2160 pixel dimensions but it is generally reserved for motion picture cameras or high-end prosumer cameras.

So, to answer your question, your Spectrum cable feed - or any cable or broadcast feed - will have to be upscaled to show on your UHD TV. But, since we're nowhere close to having UHD TV broadcasts in North America, it doesn't really make sense to wait on 4K cable broadcasts before making your 4K TV purchase, especially if you plan on taking advantage of UHD Blu-ray (which can be amazing) or UHD streaming services (which are getting quite good if you have the bandwidth). I hope this helps some. Good luck!