Home Page (official): Android 5
https://www.bq.com/es/support/aquaris-e4-5/support-sheet
Information about revive un Google(unoffical)
ROMS: Android 6.0.1
https://www.htcmania.com/showthread.php?t=1144421
Sources:
github
Pablito2020
android_device_bq_krillin