Did you try comparing the SnoopDOS log from your working A1200 installation with the A4000 log?
In SnoopDOS, does it say FAIL when attempting to open xadmaster.library 10? (I know that's what the error message says, but does SnoopDOS also say it) Is there anything else that fails?
Did you try copying the xadmaster.library from your A1200 to your A4000?
I know these are some pretty lame questions, but you have to be very specific when attempting to solve strange problems like this.