Juergen,
According to the specs, if you have a 1000Base-T mini-GBIC (J8177B) installed in one of the open slots, you must have code G.07.69 or better. Also, only one is allowed in this module. If you have any GBIC's installed, try removing them from the ports in the J4908A and see if the module will come up.
A max of 16 mini-GBICs are allowed in the switch; if the two included in the 4908 module puts you over the 16-port switch limit, this may also cause a problem. If you can take another module out containing GBIC slots, try removing it and then seeing if the 4908 module will come up. I'm unsure of what happens if you have more than 16 mini-GBIC slots, even if some of them are unpopulated - just a thought. Good luck!
Jim