First, to create 4,000 MB of swap space:

$ sudo dd if=/dev/zero of=/swapspace bs=1M count=4000

4000+0 records in

4000+0 records out

4194304000 bytes (4.2 GB) copied, 5.92647 s, 708 MB/s

or

$ sudo fallocate -l 4000M /swapspace

Next turn it into a usable swap file:

$ sudo mkswap /swapspace

Setting up swapspace version 1, size = 4095996 KiB

no label, UUID=7d1895e4-7ccf-42c6-979a-51ebddb49e91

Activate it:

$ sudo swapon /swapspace

Confirm active swap spaces:

$ cat /proc/swaps

Filename                Type        Size    Used    Priority

/swapspace              file        4095996 0       -1

Next, add the following line to /etc/fstab to activate the new swap at boot:

/swapspace none swap defaults 0 0

See also this wiki page: https://help.ubuntu.com/community/SwapFaq