PowerPoint (while it has a very nifty user interface) is listed as a
presentation software.  It shines at that, but not so at printing.

I would strongly suggest using ...
 Publisher (which has built-in 4-up Postcard templates)
 Word (which uses the page edges when laying out print jobs)

However, if you must use PowerPoint to print postcards, be aware that
PowerPoint measures within the "available print area" of a page, not the
paper size.  This may seem to be a subtle difference, but it is very
important.  The output may also be scaled to fit the available print area,
so 1 screen inch might not equal 1 paper inch.  Also note that changing the
printer (and therefore the available print areas) will change your set-up
requirements.

You could start by drawing 4 squares on a slide and print them onto regular
paper.  Hold the paper over the postcard paper and determine how you need to
adjust the squares to line up with the postcard edges.  You will need to
print these squares multiple times in order to fine tune their placement.
These squares will become the guides for where you can print on the
postcards.  You will need to repeat the 4 squares trick for the other side
of the postcard as well, since the available print area will shift if you
are using landscape printing layouts.

What Bill said ... and also have a look here:

Margins are wrong, printout is off-center, I don't want any margins
http://www.rdpslides.com/pptfaq/FAQ00097.htm

-----------------------------------------
Steve Rindsberg, PPT MVP
PPT FAQ:  www.pptfaq.com
PPTools:  www.pptools.com
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