Four ply waxed linen thread is eminently suitable for leatherwork. I'm just getting started (no points for guessing what I am aiming to be skilled enough to make!) but had a watch strap made a year ago that was built with this thread and can report that the hand stitching is still in excellent condition. The stitches are tiny, so you don't need to worry about two ply. Have a look here - http://toshi-straps.co.uk - all about watch straps, but very informative none the less.