As we all likely know, Alembics are famous for sustain. My 6 string Elan Plus has the standard 5 purpleheart laminates, and sounds great, having noticably greater sustain than most other basses I've heard; really, the only basses I've heard with comparable/superior sustain are other Alembics.
But my custom 8 string bocate Europa -featured custom June 1999- with its 6 Ebony laminates and 3 purpleheart laminates AND THE BRASS SUSTAIN BLOCK completely blows the Elan and every other bass I've ever heard completely off the map in the sustain department. I've had several times when I play it for other players and just pluck a note (especially on the B and E strings) and let it ring, and after several seconds, any listening bass/guitar players always go wow and comment on the sustain. To call it very noticable would be an understatement.
This combination produces a VERY massive neck; it's not for everyone. But if the mass isn't a problem, the sound and sustain is in a class by itself.
Anyway, if you're considering the sustain issue, make sure you consider/ask about/get educated on -the brass sustain block. I don't think this is standard on all models -at least it wasn't standard on all when I got my bass. Mica told me that this was a recommended option for people seeking enhanced sustain, and it is likely cheaper than some of the other options.
Personally, I'd recommend all of the options (combined). Someone here recommended against a 9 piece neck on a four string, but I think it would be cool... of course the laminates would be thinner, and it would be expensive.
How about a 9 laminate neck m/p/e/p/m/p/e/p/m OR a 7 laminate neck like, say, p/m/e/m/e/m/p? Either of these would look very sharp, and would -especially with the brass sustain block- likely produce some earth-shaking sustain.
Definitely ask about the sustain block.