DP Alt mode is only required in USB-C connections on the receiving display that requires it. Macs don't support DP Alt mode at all over USB-C. But its not required to maintain 60hz when going to HDMI.
Since you are going to HDMI, there's no way to support DP Alt mode on it. HDMI does not support it. Its a USB-C option only.
However as shown by leroy above, the Mac can support 60hz output for 4K.
You will want to use this adapter for it: USB-C Digital AV Multiport Adapter - Apple to maintain the 60hz over HDMI.
This adapter allows you to mirror your Mac display to your HDMI-enabled TV or display.
3840x2160 at 60Hz on:
iPad Pro 11-inch, iPad Pro 12.9-inch (3rd Generation),
MacBook Pro (15-inch, 2017 and later), iMac (Retina 5K, 27-inch, 2017
and later), iMac (Retina 4K, 21.5-inch, 2017 and later), and iMac Pro
(2018)