Thanks for all the suggestions and comments.
I really love my Thumb NT4 - it's by far my favorite bass - but I'd prefer it with a medium scale.
I asked Warwick what a 32-inch scale version would cost and they quoted me a list price of $8000! Their dealers have quoted me retail prices around $6000. That's way, way too much for me.
My alternatives are to have someone else build me a Thumb NT4 clone, or to modify mine to 32-inch scale. Either would be fine with me, as long as the price is manageable.
The problem with having a clone built is that the woods Warwick uses in the Thumb NT4 are not easily available. Bubinga, Ovangkol, Wenge and Pommele are real hard to find here.
That suggested to me that modifying my bass would be the better path.
When I bought the bass, it was in really terrible condition, and I paid a very low price, only $500. I've put another $400 into restoring it - getting the neck bow straightened out, sorting out the electronics and properly shielding it, filling the body wing separations - and now it's just about the perfect bass for me. Except for scale length. And maybe I'd like a black ebony fretboard.
I still have a lot less money sunk into this bass than it would cost to buy another one second-hand. Used Thumb NT4s typically sell for $1300 to $1700. So it makes sense to me to put a bit more money into it and get it just right. Life is short!
Even if it's another $1000, my total cost would still be less than a new standard Thumb NT4.