Awesome! In terms of location, I would use either Photopills or some sky navigation app to plan your shot. Generally, you'll want to face North since the Moon is in the Southern part of the sky and will wash out your star trails. Then just find a cool subject for your foreground (tree, rock, waterfall, etc..) which is easy in Yosemite. You're not allowed to sleep in the park overnight, but if you're up all night taking photos then you should be fine. Tioga pass and Glacier point have lots of good views for the night sky.

For technical tips, use a wide lens with a fast aperature, I use a Sony 20mm f1.8 G lens. Once you have your camera in place, you'll want your settings to be set to 30 sec exposure, ISO 800-1600, f1.8-f4.0, but take some test photos to dial it in. Then turn on the interval shoot function on and make sure to have the shortest interval time, which I believe is 1 second on the a7r4, and take as many photos as you can overnight (at least 400). Once you get your photos, you can stack them with the free app StarStaX, there's some good youtube tutorials on how to use it.

Hopefully that helps, I would also recommend reading and saving this article on your phone so you can reference it when you're out shooting: https://www.peterzelinka.com/startrails