In the past 12 months there have been lots of discussions about the Mark III versus the Mark IV.
Probably best to search for "MKIV".
This thread gives you the key differences: MkIV Differences
But I think you should also look at the threads about the Mk IV's software issues and difficulties ...
Good luck with your research
First B&O (1976) was a Beogram 1500 ... latest (2011) change has been to couple the BL11 with the BL6Ks *sounds superb*