Go to any home improvement center and buy pressure treated 6X6's. If necessary, have somebody bandsaw them down to the 4" dimension on one side. It probably won't hurt to leave them full-dimensioned both ways. The pressure treated pine will probably outlast untreated oak.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!