From original post:

Under the very old asphalt shingles on my house is 2 layers of 1/2
inch sheet board, marked "Weyerhaeuser" "insulation sheathing board"
"1B" "R1.92"

I'm guessing that the old stuff has an R value of 1.92.

The 4" foam would be much better, and keeping this old stuff really wouldn't
help much.

Sounds like you've got a good roofer.

HTH,
Vicki