To verify the issue I had to run a test. So I am having a temporary setup (do not ask for pictures...).
With only 4 speakers I could not select anything else than Centre + Fronts after powering up the system, the only speaker options acceptable by the BS 3 were 1 to 3. BS3 dedicated one pair of BL4000's as center regardless of the selected connection. Surround or back for the second set of BL4000's was not accepted, just front. After adding a third set set I could do whatever needed. I continued the test by disconnecting the centers. In this case I could play with all speaker options.
Alex was right, the option 4 was a dummy duplicated stero - what a disappointment.
Lessons learned from the test:
1. Center speaker (or a pair is needed).
2. I see no use for a BS3-setup with just center(s) and fronts. Option 4 (with BL 8000's as fronts and 4000's as rears) sounded dull (Moulin Rouge DTS) compared to a Option 5 with a pair of 4000's as the center, 8000's as fronts and 4000's as surrounds.
To complete this post, my normal setup with the BS3 is BL7-1, BL2, BL8000's and two sets of BL4000's.