The only things I know of to look for on the 400 is the cam chain tensioner, and rust around the sidestand tab.
ditto. mine rusted off the other day.
not to sound rabid, but no question here. get the 400/4. fast, fun, corners like a dream. easy to find parts for, dead simple to work on. makes a _sweet_ cafe.
expect to maybe replace battery(30), plugs(~10 each), and clean the carbs.