Screen Resolution of External Monitor

HDMI 2.0 does support 4K 60 Hz, so as long as the display also supports 4K 60 Hz over HDMI (not all 4K 60 Hz displays support receiving that signal over HDMI, fyi), then according to the specs it should work.  That said, it recently came to light that Inspiron xx93 systems that advertise an HDMI 1.4b port, which should be capable of 1440p, are somehow limited to 1080p output, for a reason that Dell has thus far chosen not to explain.  But that's the only case like that I'm aware of.

Another option since the Inspiron 7791 has Thunderbolt 3 would be to connect your display using a USB-C to DisplayPort cable, which would definitely handle 4K 60 Hz.  In fact Thunderbolt 3 offers enough bandwidth to run dual 4K 60 Hz displays with the proper dock or adapter, and the system would be able to handle up to 3 simultaneous independent displays (counting the built-in display if you keep it enabled while external displays are attached), so you could run those plus either the built-in display, or turn the built-in display off and connect a third external display via HDMI.