Apple TV

The Apple TV is not a cable box replacement.

It is a streaming box, and can only provide content to one TV at a time. You are free to move the apple TV between different TVs, but it will not magically send video to TVs it is not physically connected to with an HDMI cable.

As to whether you can downgrade your cable that is entirely dependent on what you may want to watch, and whether its available on the Apple TV.

Unlike a cable subscription, you don't get a packaged deal of content and channels directly. You can subscribe to individual providers or Apps for their content. You can subscribe to Netflix, or Hulu, or CBS all access etc...

Or there are apps like Sling that offer content packages similar to Cable subscriptions.

You would need to investigate what is available to you on the Apple TV, and 3rd party Apps, and if it covers what you want to watch for a better overall price than your current cable subscription.

If you have four TV's you would need four Apple TV's. As for content, it will depend on what you watch. There will be content that has their own subscription, like Netflix, and then Network TV apps that require authentication (you would need to look into what you watch vs what is covered by your basic cable plan). There are also live streaming services, like Youtube TV.

Can I watch TV using apps and internet only if I do not have Apple TV on all TVs?

Apple TV app Channels require the Apple TV app. Either on the Apple TV box, or natively on selected TVs, or third party streaming devices (Roku, Fire TV). That subscription content will not be available on smart TV apps other that the Apple TV app.

Devices you can use with the Apple TV app and Apple TV+ - Apple Support

Subscribe to Apple TV channels in the Apple TV app - Apple Support

Apple TV Channels FAQ: Services, pricing, availability, and everything else you need to know - Macworld

We have no idea what you watch on xfinity so we cannot recommend one way or the other.

as for your 2nd question I have no idea what you are asking.