Answer: Whenever you are designing a PCB, it is much required to keep GND plane isolated. So if in a circuit if a wire is very close to GND plane then while fabricating there will be problems. also when heavy current flows there will be undesired flow of current. Hence sufficient space has to given. "mil" is unit for distamnce usually used in pcb fabrication. 4mil is very short distance and practically when u create it will be difficult to isolate two parts. Even 8mil is very small. Thats why the manufacturer suggested you that please increase the space btw the hole and the GND layer.