Aouatif, thank you very much for looking into this.
I am actually aware that slot 4 is a 16/4 slot, but I found several posts on the internet reporting that this card works fine in a 16/4 PCI-e slot. For example
LSI 9220-8i and IBM M1015 - 2 different names for the same card.
Also the PCI-express specification requires that PCI-e host and device negotiates the highest mutually supported number of lanes on initialization. The number of electrical lanes on host and device does not need to match.
The X8, PCIe 4000MB/s specification that you found is listed under PCI Data Burst Transfer Rates. I understand this as the maximum supported bandwidth supported by this card.
Slot 1 is occupied by the graphics card, so not really a workable solution to use this slot.
Thanks again for your input and please post if you have comments or ideas.