Well, my prediction that "there will be at least three more studies on Jewish population genetics." this year certainly seems to be coming true in a hurry. The interesting thing of the paper is that they have used complete mtDNA sequences as opposed to just sequencing the hypervariable region and a few diagnostic sites. More comments to follow.
You can read more about the findings in this CNN news story. You can also read the paper via Family Tree DNA (pdf).
The lead author, Doron Behar is associated with Family Tree DNA, so expect a "Jewish Daughters" type of test to become commercially available very soon, followed no doubt by wild speculation about the identity of the four mothers of Ashkenazi Jewry and their relationships with Biblical personalities.
Am J Hum Genet (online early)
The Matrilineal Ancestry of Ashkenazi Jewry: Portrait of a Recent Founder Event
Doron M. Behar et al.
Both the extent and location of the maternal ancestral deme from which the Ashkenazi Jewry arose remain obscure. Here, using complete sequences of the maternally inherited mitochondrial DNA (mtDNA), we show that close to one-half of Ashkenazi Jews, estimated at 8,000,000 people, can be traced back to only 4 women carrying distinct mtDNAs that are virtually absent in other populations, with the important exception of low frequencies among non-Ashkenazi Jews. We conclude that four founding mtDNAs, likely of Near Eastern ancestry, underwent major expansion(s) in Europe within the past millennium.