You can't do this in either 2007 or 2010. I suspect that its nothing more than an oversight :-
For the Sunday through Saturday options then only 1/2/3/4 and last options make sense (there are never going to be 8 Mondays in a month for example)
For the day option you might as well use "Day X of every Y month(s)"
Only for the weekday/weekend day does anything > 4 make sense.
However - in SharePoint 2003 you only had the option of Sun > Sat - so by the time these were added in 2007 either no-one revisited the "First/Second...." dropdown or decided that the effort to change the available options depending on if you choose day/weekday etc wasn't worth the effort for edge cases like "Repeat on the 8th weekday of the month" - which I would think is pretty rare.