Photoshop using 94% of memory on PC with 4 photos(masked) open. Why?

A few users have had this problem, and most of them have come back reporting that a full and complete reset of preferences has fixed it.

A full reset means moving the whole settings folder out of your user account, allowing PS to build new fresh settings. This returns Photoshop to clean, out-of-the-box factory state. Remember to save out actions, brushes etc.

The thing about preferences is that it contains a lot more than your own user settings - it's the complete application configuration, including lots of hidden and interdependent parameters. Preferences are rewritten on every exit, making it prone to corruption in irregular shutdowns.

That surprises me.  I only have 64GB on my current systemn because that's what my previous system had, and it never got close to filling up.  Check the boot directory of your primary Scratch drive, and look for how big the Photoshop Temp files are, and how much free space that drive has.  I'd expect to see upwards of 30GB with four large projects open.

How much memory do you have allocated to Photoshop?
Do you have a zillion fonts and brush presets?

How long does Photoshop take to load, and does the Splash screen appear to slow down at any point?

You could try opening with Shift held down, which forces third party plugins and extensions to be ignored, and see if that makes a difference.  I am not sure if third party plugins use allocated or unalocated memory.  I suspect it is the latter.

Photoshop is not great with giving up RAM and scratch space even if you close everything.  You need to close and restart Photoshop to get back to having your full amount of free space.

I'll be interesting how this pans out.