Amar;
The E3 and E4 designation represents the version of the analysis engine installed on the sensor. The signature developers create signatures to the most current release of the analysis engine (E4 currently). Without the most current analysis engine (and an active license) you cannot apply signature updates to the sensor.
It is possible to upgrade an AIP-SSM from release 6.0 to 7.0 using the current 7.0 upgrade package (.pkg file).
Scott