Hi Aouatif,
Thanks for testing this.
I also received an answer from Dell technical support. They write that it is not supported in that slot. I suppose they mean a 8 lane card is not supported in the 16/4 slot, as your test also discovered.
Now I wonder if there is a 4 lane raid card that works in the optiplex 990's 16/4 slot.
soliemru.