This forum post should help. The moderator's post has the information you need.

https://www.dell.com/community/Alienware-General-Read-Only/Alienware-15-R2-Laptop-Highest-external-monitor-resolution/td-p/5520291

I'm skeptical of that moderator's answer.  4K 60 Hz on HDMI requires HDMI 2.0, and I've seen Dell launch systems only this year that explicitly say they support HDMI 2.0, which leads me to think that older systems that don't explicitly mention 2.0 may not support it.  And that moderator reply is from 2.5 years ago, long before Dell started advertising HDMI 2.0, so I would be very surprised if it will run 4K 60 Hz over HDMI.  The only possible way I can see it working would be if the HDMI output was wired directly to the discrete GPU, but I'm not sure that it is.

However, that system also has a Thunderbolt 3 / USB Type-C port, and that should work fine for 4K 60 Hz.  If you don't need to use that port for anything else, just get a USB-C to DisplayPort cable or USB-C to HDMI 2.0 cable.  If your display has both inputs, get the USB-C to DisplayPort because it's simpler.  The USB-C outputs native DisplayPort anyway, which means the USB-C to HDMI cables have a converter chip built in, and that just adds more cost and complexity.  But if you do need HDMI, then make sure the USB-C to HDMI cable specifically supports HDMI 2.0.  Not all of them do.  You'll be fine with any DisplayPort cable though because DisplayPort has supported 4K 60 Hz for a very long time.