Running two external 4k displays over thunderbolt to HDMI MacBook Pro (Retina, 15-inch, Late 2013)
Yes you can for the MacBook Pro (Retina, Late 2013) and later

If you use a 60Hz Multi Stream Transport display with the MacBook Pro with AMD Radeon R9 M370X graphics card only one additional Thunderbolt display is supported.

You can use 4K displays and Ultra HD TVs at the following resolutions and refresh rates via the built-in HDMI port of your Mac:

3840 x 2160 at 30Hz
4096 x 2160 at 24Hz (mirroring is not supported at this resolution)

Check out this page from Apple  https://support.apple.com/kb/sp690?locale=en_GB

https://support.apple.com/en-gb/HT206587