Hey bud,
just to let you know, I went for a 33" on my recent custom, as I have big hands with wee fingers, and it's been great!
It's a 6 string and the low B on it is fine too!
I reckon you could go to 32" for a 4 string without noticing too much of a difference in tension and tone.
Shorter scale basses tend to sound a bit plummier, i.e. less defined on the lower notes but have a much sweeter sounding upper register, something like a nice Jazz Guitar...
Reckon 32" would be the one to go for, however, probably a good idea to maybe try and find a 32" and give it a go before you start cutting into the walnut.
On the one piece neck thing, I don't know much about construction of basses, but I'd maybe use some kind of reinforcement, graphite, or steel rods or something, just as a precautionary measure...
Good luck!
Eude