AN A320 FLT CREW FAILED TO HOLD SHORT OF THE 4L APPROACH PATH AT BOS.

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WE HAD JUST MADE A NORMAL LNDG AT BOS ON RWY 22L. TWR INSTRUCTED US TO ROLL DOWN TO TXWY B; TURN R; HOLD SHORT OF 'RWY 4L APCH' AND CONTACT ANOTHER TWR FREQ. AS WE TURNED ONTO TXWY B; I POINTED OUT THE HOLD SHORT SIGN (ABOUT 500 FT AHEAD) TO THE CAPT. I THEN DID MY AFTER LNDG ITEMS; AND SWITCHED TO TWR BUT DID NOT IMMEDIATELY CALL. (I THINK I WAS STILL DOING THE AFTER LNDG FLOW AND CHKLIST.) A FEW MOMENTS LATER; I LOOKED UP AND WE WERE RIGHT AT THE '4L-9 APCH' SIGN AND STILL MOVING. I SAID TO THE CAPT THAT THIS WAS OUR HOLD POINT -- STOP NOW. THE CAPT MADE A GENTLE STOP AND OUR ACFT NOSE WENT ABOUT 50 FT BEYOND THE LINE. I THEN CALLED BOS TWR AND THEY CLRED US TO CONTINUE ON TXWY B. APPARENTLY; THERE WAS NO CONFLICT WITH ANY ACFT. THE UNDERLYING CAUSE HERE WAS THAT THE CAPT DID NOT REALIZE THAT WE WERE TO HOLD SHORT OF AN 'APCH.' HE WAS EXPECTING TO HOLD SHORT OF 'RWY 4L' AND WAS EXPECTING TO COME UPON A RWY AND THEN HOLD SHORT OF IT. HOWEVER; AT THIS HOLD SHORT POINT; THERE IS NO RWY. I AM VERY FAMILIAR WITH BOSTON AND I SHOULD HAVE MONITORED OUR TAXI PROGRESS MORE CLOSELY. THE HOLD SHORT POINT WAS WELL-MARKED; BUT IT IS AN UNUSUAL SITUATION WHEN YOU HOLD SHORT OF AN OVERHEAD APCH PATH.