I have owned, bred and shown dogs for pert near 51 years (retired now) and I have never seen a dog go for 4 weeks. Not to say that it can't happen though. I've had one come in only once a year. Basenji's will often do this too. Most bitches will stop bleeding at around two weeks and this gives owners the impression that they are out of heat when, in fact, they are not. I've never seen or heard of a dog going 5 weeks in heat unless there was something wrong.
I just wish that people who own both male and female dogs would do their homework so that accidental breedings like this won't happen. I also think that only qualified breeders should be allowed to own intack dogs. If you saw the look on the faces of all the unwanted dogs out there as they are put to death because no one wanted them, you'd agree with these words. And don't think that just because you THINK your puppies are going to a good home that everything is heavenly. I've had to take back several of my pups because they were being mistreated by buyer who swore that they were ideal owners. Just my opinion.