The CDX2 has Datalink and 16bit converter while CDX has 14bit converter and RCA plugs.
I've been looking at the manual for the double cassete version on site and as that type of Beocenter 4000 does not have Datalink you would be better of with a CDX unless you plan to go with a Datalink B&O system at some point.
Datalink is for the systems to kind of talk to each other so you would just use one touch to start a Beogram or Beocord as an example.
The CDX is a great sounding machine. Left hinge might break and some after soldering comes with the age but its nothing when you know about Beoworld. The laser is very strong and I can't recall any CDX troubles with laser faults on this forum.
It's 14bits but its a wonderfull kind of analog sound to it and many owners here me included recommends it highly. It compares very well to standards of today and outperforms them too. The highend is not as shrill. Keep in mind that CDR's is after the CDX family were born so not all players might be able to play them. My CDx does but it depends on the machine itself.