AN ACR FLC RPT ON A RWY INCURSION WHILE TAXIING AT BOS; MA.

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PARKED AT GATE; TAXI CLRNC WAS 'TAXI TO RWY 15R VIA TXWY K; TXWY L.' UPON MAKING THE L TURN TO FOLLOW TXWY K PARALLEL TO RWY 4L/22R; WE WERE TOLD TO 'STOP.' WE WERE ASKED IF WE COULD MAKE THE R TURN ONTO TXWY W TO NOW TAXI TO RWY 9. WE ACKNOWLEDGED WE COULD. WE WERE IN FACT AT TXWY W ON TXWY K. WE WERE CLRED TO TAXI TO RWY 9 VIA TXWY W AND TXWY S; TO HOLD SHORT OF RWY 4L. I DID NOT HEAR THE 'HOLD SHORT OF RWY 4L' (WE WERE IN FACT HOLDING SHORT OF RWY 4L ON TXWY W WHEN WE RECEIVED THE CLRNC) AND I CONTINUED TO TAXI TXWY W AND TXWY S TO RWY 9. HOLDING SHORT OF RWY 9 ON TXWY S WE WERE TOLD AGAIN 'STOP.' I REPLIED WE ARE STOPPED AND HOLDING SHORT OF RWY 9 ON TXWY S. THE CTLR THEN ADVISED ME THAT HE HAD TOLD US TO HOLD SHORT OF RWY 4L. I APOLOGIZED AND HE INDICATED IT WAS NOT A PROB. BEING THAT WE WERE ALREADY ON TXWY W SHORT OF RWY 4L AT THE TIME THE CTLR CHANGED OUR CLRNC; I FEEL BETTER UNDERSTANDING WOULD HAVE OCCURRED HAD WE BEEN TOLD TO HOLD OUR PRESENT POS AND THEN BEEN GIVEN THE TAXI CLRNC TO RWY 9 WHEN WE WERE CLR TO TAXI ACROSS RWY 4L. SUPPLEMENTAL INFO FROM ACN 455720: I BELIEVE THAT THERE WERE SEVERAL CONTRIBUTING FACTORS. FIRST; THE SECOND TAXI INSTRUCTIONS GIVEN WERE INCORRECT. AS WE TURNED R ONTO TXWY W; THAT IS WHERE WE SHOULD HAVE HELD SHORT OF RWY 4L. THE ADDITION OF TXWY S IN THE INSTRUCTIONS CAUSED US TO THINK TXWY S WOULD BE WHERE WE WOULD HOLD SHORT OF THE RWY 4L. ANOTHER CONTRIBUTING FACTOR WAS THAT; BY CHANGING THE TKOF RWY; I AS THE FO WAS IN THE PROCESS OF GETTING NEW PERFORMANCE NUMBERS FROM OPS ON RADIO #2. A THIRD CONTRIBUTING FACTOR WAS A NEW CAPT. THE CAPT WAS ON HIS 4TH TRIP AS A NEW CAPT. IN SUMMARY; IT IS ULTIMATELY OUR RESPONSIBILITY AS THE PLTS TO BE SURE OF OUR TAXI INSTRUCTIONS AND TO COMPLY WITH THE INSTRUCTIONS. FORTUNATELY; RWY 4L WAS NOT AN ACTIVE RWY.