Not sure if this can be done, never tried it, but if you boot the A4000 without the busboard, and you press both mouse buttons does the Early Boot Menu appear faster ?
If it still takes 30 seconds, I guess the problem is the motherboard and then the Buster chip (Bus contoller chip as said before).. Not sure how expensive Busters are these days, but I reckon they are hard to come by nowadays...
What reb. Buster you have inside the Amy ?