Do puppy's teeth come out?
Do they grow back?
Is it normal for them to loose teeth?
Should we see a vet about this?

What is happening with your pup is quite normal. As with human children, puppies teeth do fall out and their adult teeth grow in. It just happens a lot sooner with puppies than humans. This is not worth spending money on a visit to your vet. Your puppy is just fine.

Yes, routine.

Young Labs, which I know best, and other puppies tend to very bad about
Biting. You see a litter of them, and all the ones that are awake are biting
Another one or themselves. I am not even sure they realize that when they are
Alone, if they quit biting, they would quit being bitten. At 3 to 4 months
They are getting their adult teeth, and it seems they spend every waking
Moment biting or chewing. One thing you can do at that stage is to knot and wet a piece of cloth. Then freeze it. The cooling will soothe the gums. Only let the puppy have it when you are there to watch it. I maintain a Lab's favorite chew toy is another
Lab. Otherwise they settle for any person they can. They keep hoping to find
One that won't yelp, jerk their hand away, and leave.

You just have to keep on correcting them, hundreds of times, not dozens.
Provide sturdy, safe toys such as Kongs and Nylabones. Avoid things they can
Chew pieces off and choke on them. Keep them away from electrical cords.
Crates are essential for most young Labs and other dogs.

Absolutely normal, they will fall out and be replaced just like people teeth...