I currently own 3 - a 1968 4001, an early 1973 4001, and a 2009 4001C64.
The '68 has the thinnest neck I've ever played on a bass, the pickups are lower output, and it needs light strings to play properly. That said- it sounds literally like no other bass I've heard. Growliest bridge pickup I've ever heard, warm, fat neck pickup, very unique sound.
The '73 I'm pretty sure would survive a nuclear bomb. The bass is absolutely rock solid- I can put 40-95's or 55-110's on it and that neck WON'T move. LOUD pickups, balanced sound, and the neck is a baseball bat. Very solid piece of equipment.
The '09 4001C64 has an incredibly loud bridge pickup, and a very agressive tone overall. It feels like the '73, but it's not as smooth sounding.
All different, all fantastic sounding instruments.