I believe if you check with pebble tech they will say never.
What people refer to as pebble tech is not always the same, to be on the safe side go to the following link. http://en.allexperts.com/q/Swimming-Pool-Spa-2261/sand-blasting-tile.htm
Yes, but don't overdo it, judge how much you need to get the job done and rinse properly as soon as possible
YOU DONT
Pool had been resurfaced 4 years ago, should it need an acid wash or was it improperly maintaned.
It should probably leave it to a professional. They use muriatic acid.