I posted my question on the Family Tree DNA users group on Facebook. I can't encourage you enough. If you have genealogical questions, be sure to post them in the appropriate groups on Facebook. There are knowledgeable people out there that are happy to answer your questions!
I brought up this cousin's DNA in the chromosome browser along with mine. I also brought it up with my mother's and my aunt's DNA. I wanted to compare the matching segments. The segments that are interesting are on chromosomes 8 and 21.
|Chromosomes 8 and 21 between this fourth cousin and myself|
|Chromosomes 8 and 21 between this fourth cousin and my mother|
|Chromosome 6 between this fourth cousin and my aunt|
The helpful person on Facebook described these as "no calls". I had never heard this term before. No calls, in regards to DNA tests, are small segments of DNA that, for whatever reason, did not appear in the results of a DNA test. These are technically errors in the test where I inherited segments of DNA from my mother that don't appear on her test results. As long as less than 1% of your DNA results are no calls, the company considers the test valid. I'm told that if we would test at another company, it's probable that we would not see these same no calls. We'd likely have no calls at other points of our DNA.
The thing that is going to make these particular no calls challenging is that they appear in segments in common with a known cousin and makes one long matching segment with my mother appear as two shorter segments, resulting in a drastically reduced estimate of how closely she's related to people on this branch of her tree.
So, keep "no calls" in mind when you see DNA matches that don't make sense. It might not be that this person is a double-cousin. It could be that one of the people you're comparing just didn't have that segment of their DNA show up in their results.