DIAGNOSIS The HDMI handshake is failing, or the Display Resolution setting is incorrect. The Chromecast is defaulting to a lower, safe resolution.
FIX Go to Settings > Display & Sound > Resolution. Select '4K 60Hz' or '4K 60Hz Dolby Vision' if your TV supports it. The system will test the resolution. Confirm if the test screen looks correct.
IF THAT DIDN'T WORK Your HDMI cable or port may be the issue. The Chromecast requires a High-Speed HDMI cable for 4K. Try a different, certified High-Speed HDMI cable. Try a different HDMI port on your TV, preferably one labeled 'HDMI 2.0' or '4K'.
HEADS UP Some TVs require you to enable an 'HDMI UHD Color' or 'Enhanced Format' setting per port for 4K/HDR signals. Check your TV's external input settings.