First on all 4 bolts, the outer surface to the first threads were all very near 4 and 3/8". I pushed a stiff wire into all the holes and the depth on them was very close to 11". 11 inches seemed pretty deep to me. Does that sound OK
Another way to do it is make a small "L" hook on the end of the wire. Put it in the hole then pull back out till it catches on the inside of the hole in the block.