CKN
That's a Great Question
I have no idea how many miles you drive per year on your Wife's Highlander.
Since they're OEM tires with 4/32nd's tread left, they have Zero resale value, and are probably sub standard. Since you posted the question, you've obviously thought about replacing the tires.
For piece of mind, go ahead and replace the tires and don't look back. You'll feel better. Safety 1st.
My 2011 Ford Expedition EL King Ranch 4x4 has 376k miles on it. I recently purchased my 6th set of Michelin Defenders. Brand new, they come with 12/32nd's of tread. I typically run them 75k miles which leaves between 5/32nd's to 6/32nd's of an inch of tread left. Then I sell them on Facebook Marketplace for $300-$350.
Anything less than 5/32nd's makes them hard to sell.
Discount Tire/Michelin run pretty good sales twice a yr. In March/April, and September/October if you want to save a few bucks.
Safety 1st!
Good Luck