A four-gabled house seems to refer to an X-like shape, with a gable extending in each direction:
Also, take a look at this image and house-plan for a house called The Four Gables.
This is a design where, clearly, there is no possible place for a fifth gable. Each direction of the house is already taken; above the gables, the house ends in a single point.
Hypothetically, you might stretch the house out, add another story above the crossed gables ("Perhaps they would hurt her so deeply that she could at last ascend to the fifth gable"), but that would seen like very odd architecture, and less likely to be described as a four-gabled house to begin with.
Google shows me that additional configurations with four gables are entirely possible, e.g. this one, in the gardens of Burrswood Hospital:
It's conceivable that with a structure like this, it appear to be a four-gabled house from its front facade, but have additional gables in back, obscured from view. That would be a "four-gabled house" with more than four gables.
This doesn't seem to match up with the constant stress that it is a Four-Gabled House, or with the description of "ascending" to the fifth gable, but it's hypothetically possible.
Appearing and Disappearing Gables
All that being said, it seems fairly clear that the story is banking on the reader understanding the house as having precisely four gables, and the fifth gable as being magical (and inaccessible by normal means).
The constant reference to "the four-gabled house", and the matching of four women for four gables, is clear. We're not intended to question the architecture here.
Likewise, the story is titled "The Fifth Gable" -- telling us, right from the start, that there will be a fifth gable. An obvious implication is that the fifth gable is special, remarkable. But then it isn't referenced again until the very end. To me, this omission indicates that the fifth gable is not a mundane part of the house -- if you know there's a fifth gable, but keep calling it a four-gable house, that's because there's a very immediate reason not to count the fifth one.
You call attention to this very interesting snippet:
The women who lived in the four-gabled house found each other in tabloids, then in Sunday papers, then finally in a medical journal that three times failed to pass a peer review. But before then, the woman who made her bed in the third gable had lived alone. And the house had only one gable, and she could bear no children.
This could be read simply: before the women convened together, the third woman lived in a different house, and that house had only one gable.
But knowing the story's conclusion, knowing that magically-appearing gables are an option, and knowing the third woman's unique significance, does offer us an intriguing reading: rather than the fifth gable being a magical feature of the house or the coven, each woman brings a gable into existence.
We eventually learn that the third woman is the linchpin; the other women are primarily there to cause enough suffering, that the third woman will be able to give them a child. This casts the above paragraph in a very different light: not that the third woman was one among many, and lived in a different place. Rather, she had a unique ability, and gathered others around her, in order to be capable of actually putting it to use. So there is reason to see this description as significant, see the third woman as she was as being significant, and guess that it might be the same house.
The other component in this reading is: the story is deliberately misleading you. Until the end, the women appear to be of a kind -- all of them barren; none of them able to help; none of them able to carry infants to term. The story appears to be Marigold trying one after the other, all of them inevitably failing, Marigold growing more despairing -- just as the women have. This story pattern, given the title of "The Fifth Gable", might be pointing in a clear direction: that Marigold, too, will despair, and join the women, getting a gable of her own. (On my first readthrough, her constant baked goods seemed like it might make for her eventual modus operendi -- crafting children out of dough, perhaps.)
So that's an example where there's a strong implication that the women create the gables.
Further support for this can be found in the third woman's line:
“He wasn’t really mine. None of them have been.”
There's a stress here -- that even though the four women are doing all of this impossible creation of children, the mother participates as well. So, in a very real sense, the fifth gable is Marigold's -- just not in the way we (might have) expected.
In conclusion, while the simplest reading is that the house has four gables and a magical fifth one, there is also support in the story for a reading where each woman causes one gable to spring into creation. This would address the line saying "the house had only one gable".