Maybe send him an offer of about 1600 Euro, in todays market that would be reasonable. (thats about $1750, I would stay south of $2500 incl shipping on this one)

Here's one, you could probably have the fretless fingerboard permanently installed if you prefer,
 
http://m.ebay.com/itm/Alembic-Spoiler-4-String-Bass-Fretless-Stainless-Steel-Fingerboard-/151972324688?nav=SEARCH

This looks nice...
 
http://newyork.craigslist.org/mnh/msg/5454537166.html

yes it does!

Yeah very nice!
 
Upgraded electronics, most likely Elan or Essence +Q (2 preamps is Elan, one preamp is Essence) and maybe upgraded finish? (hard to see but looks like high gloss finish)
 
Info on the preamps: http://club.alembicguitars.net/Images/16271/24781.html
 
Stay away from the multi fretboard bass, unless you're prepared to spend some $$$ on the restoration.

Yes it is best to stay away from the interchangeable neck spoiler. I recall reading somewhere on the site that there is a twist in the neck due to the modification. My guess is it would be fairly expensive to bring it back to original specs and likely exceed the resale value.  
 
The Orion looks like a nice bass. I suspect it is the Essence w/Q switch package as that was a pretty common upgrade back when this bass was made. This is the same electronics I have on my fretted 5 string Orion and they work well. Looking at the hardshell case it appears to be a G&G who is the same company that makes the cases for Alembic. The price seems reasonable and is less than the street price from a dealer was in 2004. Around that time several of the East Coast dealers were selling them for $2350 with a hard sided gig bag (or would you call it a semi hardshell case?).  
 
Keith

I have an Orion 4 and the same electronics, it's a great bass! The electronics are effective and easy to use live, the bass is light and comfortable to play, and it sounds great!

An update of sorts... While that perfect $2500 to $3500 Alembic fretless 4 has yet to find me, Warwick made a German Corvette fretless 4 that is doing a standup job for me at present- it's a clone to the one that was sadly stolen. Yet the dream still must manifest: a fretless Essence 4, Fretless Europa 4, Fretless Rogue 4, or a Fretless Orion 4.... Many thanks again to people on this forum. Much Respect!
Ps. My Warwick was built mostly by robots, and that concerns me at some level. (-;/    They are not alien robots though, so I guess it's all good. I mean, hey, I support interplanetary immigration, and specifically the legal immigration of alien robots to Earth, as much as the next guy. But will American robots lose their jobs to alien robots? Only a tear in the time vortex will tell us. Until then, we wait for an Alembic to appear.

An update of sorts... While that perfect $2500 to $3500 Alembic fretless 4 has yet to find me, Warwick made a German Corvette fretless 4 that is doing a standup job for me at present- it's a clone to the one that was sadly stolen. Yet the dream still must manifest: a fretless Essence 4, Fretless Europa 4, Fretless Rogue 4, or a Fretless Orion 4.... Many thanks again to people on this forum. Much Respect!
Ps. My Warwick was built mostly by robots, and that concerns me at some level. (-;/    They are not alien robots though, so I guess it's all good. I mean, hey, I support interplanetary immigration, and specifically the legal immigration of alien robots to Earth, as much as the next guy. But will American robots lose their jobs to alien robots? Only a tear in the time vortex will tell us. Until then, we wait for an Alembic to appear.

LOL (-!

Dang!: Elan fretless

Step away from that bass Flip..... :-)

Thermy may hear you so be careful what you say about American robots.  We need a happy and healthy Thermy to help the Elves with custom pickup routing and fingerboard sloting  
 
https://www.instagram.com/p/47iQVdgNJb/?hl=en

Tyler, have you considered having Alembic do a fretless conversion on your Rogue 4? It's not cheap, but not prohibitively expensive either. It might be worth a discussion with Mica.  
 
FWIW, I hear she has a few Elven Robots working there...

Ahhhhh!!! Now the Elan fretless shows up... Wish me luck on negotiations with my financial planner! (-;
 
I hadn't considered a fretless conversion. My Rogue 4 is so sweet, hard to envision it going into elective surgery. With my preference for unlined fingerboards, I wonder what the subtle inlay options might be?