Simple question: Is it possible to use a 8Gb/s Fiber HBA card with a 4Gb/s system? Is there some technical documentation about that?
Should you mean: Can I connect with a 8Gb/s HBA to a 4Gb/s device/FC switch, then the answer is yes, without a problem, and also to a 2Gb/s device etc. It's basically the same as with ethernet: You can connect a GB ethernet card to a 10 Mbit hub if you like.
What kind of documentation are you looking for?