GND CONFLICT WHEN A C182 STARTS TO TAXI INTO POS RWY 4 AS A LNDG ACFT PASSES OVER HIM ON SHORT FINAL TO RWY 8 AT HYI.

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WE HAD JUST COMPLETED OUR RUN-UP IN THE RUN-UP AREA JUST BEFORE THE HOLD SHORT LINE AND RECEIVED AN IFR RADIO CLRNC FROM AUSTIN APCH ON THE APCH FREQ. APCH ASKED US TO ADVISE WHEN WE WERE #1 FOR DEP. WE HAD NO TFC IN FRONT AND SO ADVISED WE WERE #1. AUSTIN APCH THEN RELEASED US ON OUR IFR FLT PLAN. WE BEGAN ROLLING TOWARDS RWY 4 (RWY CONFIGN IS ODD AS THE ENDS OF RWY 4 AND (I THINK) RWY 8 COME TOGETHER IN A V) WHEN AN INBOUND ACFT CAME OVER US ABOUT 500 FT AND IN FRONT OF US BY ABOUT THE SAME DISTANCE. WE WERE QUITE SURPRISED TO SEE THIS ACFT AS WE HAD HEARD NOTHING OF HIM FROM ATC AND DID NOT HEAR HIM ON THE UNICOM FREQ. HOWEVER; WE WERE NOT ON UNICOM FREQ VERY LONG. CONTRIBUTING FACTORS: WE SHOULD HAVE 1) DONE (BUT DIDN'T) A 360 DEG TURN TO CHK FOR TFC. I SUSPECT I WAS LULLED INTO COMPLACENCY BY THE IFR RELEASE AND THE WX (THINKING ANYONE FLYING WOULD HAVE BEEN IFR); AND 2) BEEN ON THE UNICOM FREQ LONGER OR BEEN MONITORING IT ON COM #2 WHILE WE TALKED WITH APCH ON COM #1. HOW IT WAS DISCOVERED: VISUALLY WHEN THE OTHER ACFT CAME OVERHEAD. WE CAME TO AN IMMEDIATE STOP AND WAITED FOR TFC TO CLR THE RWY -- THEN A CAREFUL CHK BEFORE PROCEEDING.