The big change here is the move from HDMI 1.4b to HDMI 2.0.

HDMI 1.4b:

• 4096 × 2160 at 24 Hz
• 3840 × 2160 at 24, 25, and 30 Hz
• 1920 × 1080 at 120 Hz

That last is also known as 1080p, or Full HD.

HDMI 2.0:

• Requires special higher bandwidth certified cable
• 4K video at 60 Hz with 24 bit/px color depth

Note that there’s also an HDMI 2.1, requires still another specialized cable, for 4K at 120 Hz, and beyond. Apple’s updated dongle is rated for HDMI 2.0.