I'm not positive about his fur. it really depends on the dog. In the grooming salon I work at we've seen dogs get their whole coat back just like it was, and seen dogs with bald patches that will never come back. Luckily, since he is four years old you have a much better chance of his coat coming back normal then if he was a puppy.
What I wanted to chime in to say though is never ever go back to that groomer. Any groomer who will shave a dog without explicitly asking an owner first isn't one I would take my dog to. Sometimes a dog does need to be shaved because of mats or whatnot but I know at the salon I work at we make owners sign a completely different form confirming that they know and understand that we have to shave the mats out and another signature confirmation if we decided to shave the whole dog to make things look even after mat shave-outs.
We hear tons of horror stories of people going to cheap groomers in this area with no training expecting a Yorkie or Schnauzer cut and coming back with a naked dog. I am a firm believer in practicality over vanity (specifically I can't stand people who have long-haired dogs and keep them long who won't brush them out on a regular basis, then get mad at us when we say we have to shave all the mats out). However a good rule to follow is if a groomer is ready to just take your dog back, without discussing at all (or taking notes if its a busy salon!) the specific cut you want, then that's a big red flag and a sign to walk right back out and find someone different.
