AN A320 PIC TAXIES ACROSS RWY 4L WHEN TOLD TO HOLD SHORT AFTER LNDG ON RWY 4R AT BOS; MA.

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LANDED ON RWY 4R IN BOS. CLRED THE RWY ON TXWY Y. TWR ADVISED US TO HOLD SHORT OF RWY 4L. AS I APCHED RWY 4L; I CHKED TO SEE IF RWY 4L WAS CLR AND THEN CHKED R. I NOTICED A PUZZLED LOOK ON THE FO'S FACE AS I CROSSED RWY 4L. HE STARTED TO MENTION WE WERE TO HOLD SHORT OF RWY 4L; BUT I HAD ALREADY TAXIED ACROSS THE RWY. ONCE I CROSSED RWY 4L; I DID SEE A BEECH 1900 TAXI INTO POS ON RWY 4L. TWR ADVISED US WE CROSSED RWY 4L WITHOUT A CLRNC. WE TAXIED TO THE GATE WITHOUT FURTHER INCIDENT. SUPPLEMENTAL INFO FROM ACN 464400: I READ BACK 'HOLD SHORT RWY 4L.' AS I COMPLETED MY AFTER LNDG FLOW AND WAS PICKING UP MY RWY DIAGRAM 10-9 PAGE; I LOOKED TO SEE RWY 4L APCHING. I LOOKED TOWARD CAPT AND SAID 'HOLD SHORT; 4L.' CAPT REPLIED 'THAT IS 15' AND CONTINUED TO TAXI ACROSS RWY 4L. AS WE TURNED S ON TXWY T; TWR ADVISED 'I SEE YOU CROSSED RWY 4L; YOU WERE ADVISED TO HOLD SHORT; CONTACT GND 121.9.' WE TRIED TO CALL TWR CAB ON LANDLINE BUT NO ANSWER. CORRECTIVE ACTION: I SHOULD HAVE SAID 'STOP' AND STOMPED ON THE BRAKES AND ADVISED TWR OF OUR POS.