Mn... this is one where you have to decide what your own moral compass is when it comes to dogs.
I have no issues with dogs living in kennels, esp breeds like labs who are generally less human-needy than some other breeds, but I've never liked the idea of breeding bitches being boarded out (or whatever you want to call it) and then rehomed at the end of their breeding careers. But that's me, and provided the bitch is well cared for and has a fulfilling life I wouldn't get on my soapbox about it: I just don't like it, it's not how I'd handle a dog, though it's preferable to gruesome puppy farm conditions, and also better than commercial facilities where the bitches are bred 4 times with pedigree litters that can be tracked on the KC website, and a couple of other times with either unregistered pedigree litters or to produce trendy crosses, and then rehomed at 6 or 7 (and get to go into a house for the first time ever).
The fact that any dog you had from this breeder would be trained, and if he (or she) is producing working gun dogs will also have been trained for work and proven in the field, is somewhat reassuring: this bitch will have seen the outside world, been about the place in the back of a pick-up, and had some fun in her time. People don't usually buy a puppy as a working prospect if the dam and sire don't both work (though I have known it happen when the dam was not in work but the sire was a good one).
As for number of litters, it used to be that the KC would register (I think) 5 litters, but now they will only register 4. It's not great for the size of the gene pool for a bitch to have lots of litters (just as it's not great for a stud to be used eleventy-billion times), and people I know who breed take between 1 and 3 litters off their bitches. Four litters is IMHO not ideal, but also a long way from the end of the world if the bitch is fit and well, properly looked after and given a year off between each litter - that is sensible. I have a dog from a third litter, and the bitch recovered very well and very fast afterwards despite being at the top end of the breeding age-range.
Licensing used to mean that the breeder was a puppy farmer. Now anyone who breeds is supposed to be licensed and the stipulations are pretty exacting in terms of physical well-being.
The one question I would ask is whether the bitch has been health tested or not, especially whether she has been hip scored. Dodgy hips can be expensive or difficult to fix if they cause issues in early life, and predispose a dog to arthritis and disability in later life. If the breeder is has an all-bells-and-whistles licence, I think that mean that breeding stock will have been health tested, but in your shoes I'd check that. A total hip score of below 15 below (say 7/7) is okay, a score below 10 is good, a score of 0/0 is perfect.
Also bear in mind that this would be a working line lab (albeit and adult not a loony puppy or wild adolescent) so she'll be likely to have more energy and drive than a pet-bred lab of the same age. I know a fair few working-line labs who are great pets, but they require more of their owners than do the really placid pet/show ones. Still easier than working spaniels or HPRs though.
That was longer than planned - hope it helps you clarify your thoughts.