I personally think 4GB sticks would be pushing it as officially X48 maximum memory per DIMM slot was 2GB , however P45 expanded this to 4GB per slot and along came some speculation that this also meant X48 supported 4GB per slot too, just at the time the boards were ratified,, by Intel, ASUS, etc 4GB modules did not exist or were not available so they did not test the X48 with 4GB DIMM's , hence no official support
I have read that quite a few people have had mixed luck with 4GB DIMM's, some can run 8GB (2x4GB), others have even managed to get 12GB working (3x4GB), however the magical 16GB seems VERY DIFFICULT to obtain stability with on X48 and you can forget about Overclocking with it
John