What is the minimum internet speed required for Apple TV 4K?

No minimum speed is established by Apple as being "required".

Though less than 10Mbps will cause the Apple TV to buffer a lot. Specially if trying to load 4K streams.

Apple recommends at least 25Mbps for 4K content and at least 10Mbps for HD content to avoid buffering.

Some services like Youtube will adjust quality to the available bandwidth automatically. Others will not and will buffer if available bandwidth is not enough to keep up with playback.

click here ➜ Find and watch movies with 4K, HDR, Dolby Vision, or Dolby Atmos in the Apple TV app - Apple Support

What Phil0124 is explaining is there is a difference between internet speed and Wi-Fi coverage within a home. The speed you buy from your ISP only guarantees how fast the signal is when it enters your home and reaches your modem.

Wi-Fi coverage is different and more important than speed with streaming devices (Apple TV box), smart phones, computers, tablets, Alexa devices, smart home items like lights, doorbells, locks, etc..... The coverage of your signal is affected by items such as:

• Home size (sq. footage)
• Multi Story homes
• Home construction type (brick, wood construction, metal constructions etc...)
• The amount of Wi-Fi devices within the home (tv's, computers, tablets, phones, smart plugs, smart home devices (locks, lights, fans, garage door openers, sprinkler systems etc....)
• Etc.

So when researching you need to assess your needs prior to buying. And if you are not sure of the signal strength in your home, use a tool such as iStumbler to measure in different areas of the house. If you find the signal strength is low in parts of your home that you need coverage you may be an excellent candidate for a Mesh network. If you don't know what a Mesh network is, please Google the term and you will find many many articles that explain what they are. Mesh networks are available from Costco, Best Buy, your local computer store, Amazon, etc....