The 'double' minor version increment is to align this with the versions used by JDG and WFK in an attempt to have less maintenance branches. From a 6.4.0 perspective this should not matter since this is new in 6.4.0 and the changes are minimal.
Block ER2?
It is OK, EAP 6.4.0.ER2 contains hibernate search 4.6.0.Final-redhat-1