From the first link:

  Global  16326  G=0.95075  C=0.04925
  European  12074  G=0.97896  C=0.02104
  African  2816  G=0.9052  C=0.0948
  Asian    108  G=0.630  C=0.370

So, roughly 5% of the population is affected, lower risk for European ancestry, much higher risk for Asians (37%, but the sample size is rather low).

Is the African referenced from samples from Africa or from African-Americans?

Africa (continent), there is a different row for African Americans. And "African (Other)"

Thank you very much for sharing these links, a SNP id and browser link was exactly what I was looking for!

Pardon my ignorance - what does it mean to have SNP rs1983891 (what pops up when I search FOXP4 in my Promethease report) but not rs9367106? I have only rs1983891(C;C).

Probably no relevance to the topic at hand. This research applies only to the documented SNP.

Unrelated, you may have some increased risk for prostate cancer: https://www.snpedia.com/index.php/Rs1983891

You're probably right. Look at figure 3 in the paper. The effect is for region upstream of FOXP4. The reported Prometheus snp ( rs1983891 ) is not in this region, it is in an intron in FOXP4.

Interesting. Glad I don't have a prostate then! :)