Re-sizing a Virtualbox disk image in Linux

One of my VirtualBox virtual machine had its disk full, I thought a simple resize should fix the problem. And I went:

vboxmanage modifyhd u140464-02-disk1.vmdk --resize 30000
0%...
Progress state: VBOX_E_NOT_SUPPORTED
VBoxManage: error: Resize hard disk operation for this format is not implemented yet!

What?!
Turned out "modifyhd" only support .vdi, not .vmdk. OK I will just use .vdi then:

vboxmanage clonehd u140464-02-disk1.vmdk u140464-02-disk1.vdi --format vdi
vboxmanage modifyhd u140464-02-disk1.vdi --resize 30000

Then detach vmdk disk from the VM and attach the vdi.
But after I boot it up, the disk was still full. The disk size wasn't changed. After some googling I found I need to install gparted and use it to grow the partition, and gparted has to be run at boot. Too much hassle.  All I want was some additional disk space. So I did it my way.
(Inside the VM)

sudo fdisk /dev/sda

then enter p, it showed:

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      499711      248832   83  Linux
/dev/sda2          501758    33552383    16525313    5  Extended
/dev/sda5          501760    33552383    16525312   8e  Linux LVM

Enter n to create a new partition, press enter 2 times to accept default partition size. For some reason, there was a stray segment, so I have to enter n again to create a new partition. 2 partitions were created, after a p, my partition table looked like this:

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      499711      248832   83  Linux
/dev/sda2          501758    33552383    16525313    5  Extended
/dev/sda3          499712      501757        1023   83  Linux
/dev/sda4        33552384    61439999    13943808   83  Linux
/dev/sda5          501760    33552383    16525312   8e  Linux LVM

sda3 is useless, so I deleted it by a 'd' followed by a '3'. sda4 is the one I will use. Enter 'w' to write to disk. Then reboot the VM.

After reboot, do a:

sudo mkfs.ext4 /dev/sda4

My /opt was not used, so I just mount sda4 as /opt

sudo mount /dev/sda4 /opt

Add "/dev/sda4 /opt ext4 defaults 0 2" to the end of /etc/fstab so it was mounted automatically at boot.

In the end, I should have just add another virtual disk without bothering resizing the current one.