6gb should work, so should 3.5gb. Its due to subtle differences in the memory map between the two virtual platforms. If you have more, give it more.
You may find it panics again when you go to 9.1. If so, add serial ports to the vm:
#If these are missing it can get stuck
vboxmanage modifyvm "$name" --uart1 0x3F8 4
vboxmanage modifyvm "$name" --uart2 0x2F8 3
While you're at it, the ioapic setting is often missed:
vboxmanage modifyvm "$name" --ioapic on
Then boot to the vloader and set the console variable:
setenv console vidconsole,comconsole
Then it should come up.
If this post resolved your issue, help others by selecting ACCEPT AS SOLUTION or adding a KUDO.