I have a rescue now that I am working with that has stage 4 heartworms. He was neutered before treatment.
I'm not sure the specifics, but if you want (and are able) please pm or post what the vet said to cause it.
One thing that I have come to realize is how traumatic the heartworm shots are to the system. The farther along the dog is, the more the risk of heart attack and pulmonary embolism.
The dog that I am working with (shepherd mix named Charlie) was put on prednesone for inflamation and coughing, and heartguard, and the vet had me give a few days of aspirin (as a blood thinner) to help keep things moving
Now, I am not an advocate of giving dogs aspirin, and neither is this vet. However, due to an increased risk of heart attack with this dog, the benefit outweighed the risk.
Basically the idea is that it will take a long time for the dog to be free of heartworms. BUT this way is less traumatic. It will cause some cell and tissue damage, but lung tissue heals very rapidly, and cells will repair/replace. A stage 1 or 2, I guess I will consider the shots or other methods, 3 or more, never. And even with the 1 or 2 stage, I probably would still not do the shots. Long term is best.
The way that the vet I am working with now (so far, I am really impressed, which says a lot, because I think ALL the vets I have met thus far are one neuron short of a synapse) described the shots to me, was that if you ask a vet to treat your expensive, high end show dog for heartworms, they will explain that the shots are going to be a huge risk, but will clear it up quicker, but the heartguard or other preventative will actually clear it up as well, and be much safer, but it will take about a year or so until the dog is free or nearly free of heartworms.
Ask that same vet how to treat a shelter dog or mutt, the vet will tell you to take the shots. Why? Because if the shelter dog dies in treatment, there will always be another rescue to fill it's place, and be one less dog put down due to no space. Feed and upkeep on a dog is more expensive than the shots, also. Therefore, for rescues, the LESS expensive choice is the more expensive shot. (not to mention there is no chance for the owners to forget about giving the heartguard monthly)
Now, before anyone gets upset with me, this is the point of view that a vet himself told me HE looks at the heartworm situation. THIS is how HE rationalizes treatment with a riskier method. If this is how ONE vet looks at it, well - it's worth thinking about in my book.
So all that said, anytime I have a heartworm case I am fostering or otherwise, I do the monthly heartworm medicine. Slow and steady, not fast and furious.
OH, and I am not a fan of the medication you mentioned - too easy to OD a dog on it, and kill them. There ARE safer methods of treatment, that can be done at home, without the risks of OD'ing a dog on it.
When a flower doesn't bloom, you fix the environment in which it grows, not the flower.