lots of people suggested the free edition of eagle - but it might not
be suitable because the max area supported is 100mm x 80mm - which is
a bit smaller than the 4" square the OP is looking for.

Another possible suitable tool is gEDA (http://www.geda.seul.org/)

Also, check the archive. There are loads of threads about this. Two
recent ones of note are "CAD package alternative to Eagle" and "whats
the best PCB design software around"

Jon
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