Will this damage my PC?

No. If the power button is pressed for more than a 4 seconds the pc just turns off. It is a feature.  

Actually, it can do, the 4-second power-off is a failsafe for when the computer stops responding.

I've only ever had 2 hdd corruption problems (both requiring complete reformat/ reinstallation of the OS) and both came after the power was suddenly cut to the PC and both happened to be during intensive disk access.

Admittedly the problem isnt as bad as it used to be - i remember my old amstrad 1640 with its four-platter 20mb (yes, mb not gb) hard disk drive that you had to manually park the heads, almost all modern hard disks have much higher fault tolerance than these old ones and all auto-park the heads away from the disk platter when the power goes, however there is that brief instant when a file is being moved when the File allocation table, the physical location of the file are not the same - if the power goes then, its bye-bye data.....

WOW! I remember having to park the heads. That was a very very long time ago. My first hard drive was a Seagate 10MB drive with an access time of something like 120ms.

IT will not damage the computer, but you do have a small chance of it corrupting data on the hard drive.

Strangely after this incident I notice that my PC actually doesn't have the 4 sec power off fail safe feature as the PC immediately shuts itself when my niece press the power button.