How do I enable hardware virtualization technology (VT-x) for use in Virtualbox?

Before changing BIOS settings we may want to see if hardware virtualization (VT-x for Intel, AMD-V for AMD processors) is supported by our CPU.

From a terminal issue

grep --color vmx /proc/cpuinfo ## for an Intel processor
grep --color svm /proc/cpuinfo ## for an AMD processor

If virtualization was supported the flag vmx (for Intel CPUs) or svm (for AMD CPUs) will be colored.

In case the CPU supports hardware virtualization, we need to enable it in the computer BIOS to be able to use it.

Also read How to determine if CPU VT extensions are enabled in bios? for other approaches.

Enter the BIOS (often pressing Del or F12 while booting) and see with the manual how it is named there. Each BIOS appears to have a different name fror this. Search for Virtualization, Virtualization Technology (VT-x), SVM, VMX, or similar, here shown for an Award BIOS:

For an example screenshot of an Asus EFI-BIOS see this answer on SU.

Set this entry to Enabled in case it was not done yet. Some BIOS need a cold boot (i.e. boot from power off state) to be able to use hardware virtualization.

Hardware virtualization can then be chosen in the virtual machine's System -> Acceleration settings:

By this the VM uses hardware virtualization for optimal performance. We would even be able to run a 64-bit guest OS on a 32-bit host for testing.

I had the same exact error, and fixed it by dropping the 4095 GB to 3072 MB (3 GB). This is because it was 32-bit. I think the more than 3 GB is throwing it off.