Having Optional in function parameters

[font=Tahoma]How do I go about setting up a function to optionally take a parameter? For example, to call a function like the following[/font]
[font=Tahoma]Both of these would call the same function, but parameter 4 is optional. I have done this in VB.NET with ‘Optional’ and I can default the missing parameter. Is this possible in AS?[/font]
[font=Tahoma]the skOObs[/font]