PACKMULE: I measured my lace with a caliper this morning and it looks pretty close to 4 mm diameter. I see REI has "accessory cord" in 4 mm. Using that with heat shrink on the ends would probably be a good solution.
25C: The Spyder Line looks pretty good too and comes in 3.8 mm. 1860 pound breaking strength...hmmmm might just work...lol.
I wonder if it is good to have stretch in your boot laces or not. I can see pros and cons to low stretch laces.