@rebel,
I can't help much since I have yet to try 4GB on my 4CoreDual-VSTA. However, I have had problems getting the board to boot with the second DDR2 ram populated (2x1GB config). In the end it was a question of (1) really truly emptying out my CMOS (PSU off and drained of residual, CMOS battery out, jumper shorted for several minutes) and (2) extreme, extreme care & precision installing the ram stick in the second slot. I still had black screens etc., but persevered (many reboot attempts) and eventually the bios recovery kicked in and booted, allowing me to set bios defaults and reboot again, then adjust bios settings properly.
I am of course assuming you have also already tested each stick individually in slot 1 with memtest+, a sine-qua-non.