What could they possibly do to a solid body swamp ash body with a rock maple neck to make it "worth" $4500?
Lots of things. If you were to give the same piece of swamp ash and rock maple to Fender, Roger Sadowsky, and Michael Tobias, you'd wind up with 3 very different instruments. It's really up to the player to decide which instrument feels the best in your hands, and sounds the best to your ears. Everything else is secondary as far as I'm concerned.
I hope you live near a store where you can try some of these "$4500 basses." Some people react enthusiastically to (for example) a NYC Sadowsky, and others are very meh. Each opinion is valid (IMHO).
We all need to remember this when we start GAS-ing.