According to the release notes:
"Features and Enhancements
The following are the features and enhancements introduced in the 6.4.2.0-4.1.1.0 release.
Support for New Access Points
In the 6.4.2.0-4.1.1.0 release, Instant software is introduced on the IAP-200 series and IAP-210 series devices."
There is no public release before 4.1.1.0 for the IAP 215 (IAP-210 series) and IAP-205 (IAP-200 series).