Whoof ! you're right it is hard to see. Luckily there is something you can do. Go to Finder, then View, and choose View Options (or press Cloverleaf and J), There you can make the icons bigger and change the font size (but not the font )
and how things are stacked/sorted. As for the dongle, yeah...should work. Either USB-C or DisplayPort (or MiniDisplayPort) to High speed HDMI should give you 4k, I trust you when you say your monitor only has HDMI...doesn't seem quite possible, HDMI only, but that's all I've got to go on. ((don't know the model of the monitor for sure, so can't say what it does and doesn't have). Just thought I'd mention the whole Hi Speed HDMI thing. Apparently there is a difference, standard HDMI gives you 1080 and that's it, and me being a cheapskate, and not wanting to pay brick n' mortar prices for an HDMI cable, Hi speed or standard, I found another source *cough*monoprice*cough*cough* Gee-must be dry n' dusty in here. There's also the whole HDCP thing as well. The faster cables support the newer HDCP 2.X standard , and as is par for the course, everything must support that standard or else...no 4k. Not a big problem with a monitor and a Mac pro. But...when you get an older reciever with HDMI 1.4, and a couple of modern game consoles, and the previous rule applies, it means that you can't run the hdmi through the reciever and let it do the switching. at least not if you want 4k. The hdmi goes directly to the tv set, and the audio goes to the reciever... not a huge problem, but a pain in the butt for sure.
anyways that's all i got
John B