I believe more zoos keeping Gorillas are being asked to house a breeding group and a bachelor group if they have the room or more than one group. The current males(2 not 4?) in the Stuttgart nursery are still very young though, so not sure about the authenticity of that part, but there are older young males in other European groups, including 2 in Stuttgart's breeding group and 3 others in Romagne's breeding group, any of which might be involved in the new male set-up there.
I have this info of Romagne's Curator ... He speak me of 4 young males of Stuttgart so it's possible : 2 of the nursery and 2 of the breeding group ...
In 2012, "Ya Kwanza" and the 4 young will move to Amnéville Zoo.
I have heard the same too. I think its possible that after an operation for testicular cancer(and removal of one of them) a few years back, that he may be infertile and no longer able to breed. He has only ever bred with one female in their group anyway.
By deduction, I assume you meant this observation to be about Ya Kwanza?