I need to add a sub-panel about 40 feet away from my main panel. The
previous owner left an empty 3/4 inch run of conduit in case anyone
wanted to add a sub-panel to the other side of the house (for an
addition) (conduit runs interior sub-panel will be interior). Now is
the time for that addition. I would like to run #6 wire to the sub
panel because one of the new branch circuits will have a 30 amp steam
unit for the bath. Ideally I'd like to run #4 wire even if I can.
The 40 foot run only has one 90 degree bend right at the main panel
and I could put a large pull box at the other end prior to bending
into the sub panel with any size pipe I want.

Can I get three #4's in 3/4 inch thinwall or must I use #6? My
intention was to undersize the feed breaker in the main panel to
accomodate whatever wire I can get into this pipe.