TomR wrote:

I would also be interested in knowing what the purpose of doing this is
(such as to put in a pipe etc. in the 4 inch trench), and if you will be
patching/replacing the asphalt after it is done.

If it's just to run wiring or a pipe under a sidewalk, the OP might be able
to hydraulic his way under the pavement:

http://forums2.gardenweb.com/forums/...341731539.html

There are commercial machines, but with a little PVC and fittings you can
easily make it through in most soils.