Latest version is 4.5.x Checkout the changelog
Third number, or build number, is not outlined here.
To download, you will need to have valid licence
If you want to upgrade from very old JomSocial version, a tier upgrade will be required.
For instance, it is not recommended to upgrade from 3.1 to 4.5 because installer wont install database changes in versions that are released in between (3.2, 4.0, 4.1, 4.2, 4.3 and 4.4)
Before downloading older versions, please read how to do a proper tier upgrade