How is 4K different from high-definition monitors?

Simply put, you're going to get even better picture quality. Typically, 4K Ultra HD refers to 3840x2160 pixels, which has exactly twice as many pixel rows and columns as 1080p (1920 1080 pixels) for high definition.

What kind of HDMI cable do I need for 4K?

You will need HDMI 2.0. The maximum bit rate increase to 18 billion bits per second supports new formats such as Ultra High Definition 4K video at up to 60 frames per second.

What is the best size for a 4K monitor?

Look for something in the range of 24 to 30 inches.