ACR FLC STARTS TO CROSS BOS RWY DURING TAXI OUT AND THEN AFTER QUESTIONING TWR; WAS TOLD TO HOLD SHORT. 1 MIN LATER; AN ACFT LANDED ON THE RWY.

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IT WAS THE FIRST TRIP TO BOS FOR BOTH CREW MEMBERS. WE HAD PLANNED A TKOF ON RWY 4L. AS WE TAXIED OUT; THE RWY WAS CHANGED TO RWY 9. WE WERE TO HOLD SHORT OF RWY 4L BEHIND ANOTHER ACFT; WITH CLRNC TO RWY 9. WE WERE TOLD TO SWITCH TO TWR; WHICH WE DID. THE ACFT AHEAD WAS CLRED TO CROSS. EARLIER; WE HAD BEEN TOLD TO FOLLOW THAT ACFT TO RWY 9. SO WE BEGAN TO ROLL FORWARD; BUT WE WERE UNSURE OF OUR CLRNC. I TOLD THE CAPT I WOULD GET A CLARIFICATION FROM THE TWR. THE FREQ WAS VERY CONGESTED; AND BY THE TIME I GOT THROUGH; WE WERE TOLD TO HOLD SHORT OF RWY 4L. BY THIS TIME OUR SLOW ROLL FORWARD HAD ALLOWED THE NOSE OF THE ACFT ACROSS THE HOLD LINE FOR RWY 4L. WE STOPPED WITH THE NOSEWHEEL JUST SHORT OF THE LINE. ABOUT 1 MIN LATER; AN ACFT LANDED ON RWY 4L. WE WERE THEN CLRED TO CROSS RWY 4L; AND SUBSEQUENTLY CLRED FOR TKOF ON RWY 9 FOR AN UNEVENTFUL FLT TO SEA. UNFAMILIARITY WITH THE ARPT WAS CERTAINLY A FACTOR IN THIS INCIDENT. UNCLR TAXI INSTRUCTIONS STILL LEAVE ME WONDERING WHICH OF 2 POSSIBILITIES IS CORRECT: 1) WE WERE INDEED CLRED TO CROSS RWY 4L; BUT BECAUSE OF OUR SLOW PROGRESS AND INABILITY TO CONTACT THE TWR QUICKLY; THAT CLRNC WAS THEN RESCINDED DUE TO THE PROX OF THE ACFT ON FINAL; OR 2) WE WERE TO HOLD SHORT OF RWY 4L; IN WHICH CASE THE EARLIER INSTRUCTIONS TO FOLLOW THE ACFT AHEAD AND TAXI TO RWY 9 LED US TO IMPROPERLY BELIEVE WE WERE CLRED TO CROSS THE RWY. IN CONCLUSION; THERE WAS NEVER ANY DANGER TO THE ACFT OR PAX. HAD TWR THOUGHT OUR PROX TO RWY 4L HAZARDOUS; HE HAD AMPLE OPPORTUNITY TO SEND THE LNDG TFC AROUND. IN THE FUTURE; HOWEVER; IT WOULD BE WISE FOR US TO LEAVE THE BRAKES SET UNTIL OUR RWY XING CLRNC IS CERTAIN.