AN MD80 CREW AT BOS FAILED TO HOLD AT THE RWY 4L 'B' HOLD SHORT LINE CAUSING A GAR. BOS TWR NOTIFIED CREW.

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UPON APCHING SPOT X ON THE BOSTON RAMP; THE FO CONTACTED BOS GND CTL FOR TAXI CLRNC. IT TOOK SEVERAL ATTEMPTS TO MAKE CONTACT DUE TO RADIO CONGESTION. GND INSTRUCTED US TO TAXI TO RWY 9 VIA TXWY K; TXWY W; TXWY S. I (CAPT) DID NOT COPY THE HOLD SHORT CLRNC. UPON GIVEN CLRNC; I TYPED INTO THE FMS SCRATCH PAD 'KWS.' THIS WAS A SHORT TAXI AND THE FO WAS BUSY COMPLETING TAXI AND DELAYED START CHKLISTS. UPON APCHING THE RWY 4L HOLD SHORT LINE; I BEGAN TO SLOW AND ASKED THE FO IF WE WERE TO HOLD SHORT 'HERE.' THE FO RESPONDED 'NO; HOLD SHORT RWY 4' AND POINTED TOWARD RWY 9. THE FO DID NOT STATE L OR R. WE CROSSED THE APCH END OF RWY 4L AND HELD SHORT OF RWY 9 ON TXWY S. ONCE WE WERE AT CRUISE ALT; ATC TOLD US TO CONTACT BOS TWR UPON LNDG. UP TO THIS POINT WE HAD NO IDEA THAT WE HAD A RWY INCURSION. ONCE THE FLT WAS OVER I CONTACTED BOS TWR; TWR ADVISED ME THAT WE DID INDEED HAVE A RWY INCURSION THAT CAUSED AN ACFT TO BE SENT AROUND. TWR ALSO ADVISED ME THAT THEY WOULD FILE A PLTDEV. CONTRIBUTING FACTORS: 1) POOR COM BTWN CAPT AND FO. WE SHOULD HAVE USED BETTER TERMINOLOGY REGARDING RWY 4L AND RWY 4R. ALSO; CAPT SHOULD HAVE VERIFIED IF ANY HOLD SHORT CLRNC WAS GIVEN SINCE I DID NOT HEAR ONE. 2) CAPT WAS ZZZ BASED FLYING AN XXX ROTATION. CAPT NOT AS FAMILIAR WITH THESE OPS AND ARPT GND OPS. 3) EXPERIENCED FO. CAPT RELIED TOO MUCH ON FO'S KNOWLEDGE OF GND OPS IN BOSTON. 4) SHORT TAXI. FO VERY BUSY WITH CHKLISTS WHICH AFFECTED HIS SITUATIONAL AWARENESS. 5) RADIO CONGESTION. 6) CONFUSING RWY LAYOUT AND TXWYS. 7) POOR ARPT SITUATIONAL AWARENESS. IN CONCLUSION; POOR COM BTWN MYSELF AND THE FO WAS THE PRIMARY CULPRIT TO THIS DEV. I FAILED TO LIVE BY THE AXIOM -- 'WHEN IN DOUBT; ASK.' THE PROB WAS THAT I RELIED ON THE FO'S EXPERIENCE IN BOSTON AND HIS SITUATIONAL AWARENESS THOUGH HE WAS BUSY WITH CHKLISTS. I TAKE FULL RESPONSIBILITY FOR THIS UNFORTUNATE EVENT.