Dependent on the software you're using, encoding of the video, throughput of the network/storage device, etc. but the Pi 4 has hardware decoding support for H.2

Dependent on the software you're using, encoding of the video, throughput of the network/storage device, etc. but the Pi 4 has hardware decoding support for H.265 up to 4Kp60. I've had no issues playing back 4K @ 30Hz using LibreELEC on a Pi 4.