Thanks for reaching out. To confirm, what type of Chromecast device do you have?
All Chromecast devices require a TV with High-bandwidth Digital Content Protection (HDCP) 1.3 or higher built-in. HDCP 2.2 is required to support 4K content. You can check this support website article to learn more about streaming 4K Ultra HD content. Also, please select the quality settings in the streaming app itself.