Upgrade BIOS, AT489AV with 786G2 motherboard

https://www.ccleaner.com/speccy

This is BIOS update just exactly for version 786G2 motherboard:

https://whp-aus2.cold.extweb.hp.com/pub/softpaq/sp53501-54000/sp53698.exe

Remember to unload antivirus, disable windows update service, disable system center service.
Once you extract sp53698.exe launch
C:\swsetup\SP53698\HPQFlash\HPQFlash.exe

After this upgrade the 786G2 motherboard still NOT support 4GB RAM modules.
Only 2GB modules is supported which brings together possibility to install 8GB at max.
Just like in brochure: https://h10057.www1.hp.com/ecomcat/hpcatalog/specs/provisioner/05/AT489AV.htm

2014.04.10 updated:
VH648AA (HP 4-GB PC3-10600 (DDR3-1333 MHz) really works on this AT489AV machine.
2014.06.18 updated:
Also 4Gb BLS4G3D1339DS1S00 are welcomed!
and by default it uses 2Gb 8JTF25664AZ-1G4D1