run a script every 4 hours with task scheduler
In Windows Server 2008/2008R2 you can set task to repeat every number of hours you want, corresponding drop down menu just present you with 1 hour option to select, but you can type in any number of hours you need (see screenshot below). 
You could just set it up to run every hour, and at the beginning of the PHP script, check the current time, and exit if the current hour modulus 4 wasn't zero.
Where it says "repeat task every", you may also just be able to change "1 hour" to "4 hours" (it's not in the dropdown, so just type it in). You can do this in Windows 7, I'm not sure about Windows Server 2008.