A B757 FLT CREW TAXI ACROSS RWY 4L AT BOS AFTER THEY WERE TOLD TO HOLD SHORT.

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UPON LNDG AT BOS; RWY 4R; TWR CTLR ADVISED US IF WE COULDN'T MAKE THE RWY 33R TURNOFF TO EXPEDITE TO THE END TO EXIT RWY. COPLT LANDED THE ACFT; AND MADE A MAX EFFORT TO SLOW THE ACFT ENOUGH TO MAKE THE RWY 33R 90 DEG TURN. I ADVISED TWR THAT WE HAD TURNOFF ASSURED; AT WHICH POINT HE HURRIEDLY ADVISED US TO HOLD SHORT OF RWY 4L AND MONITOR 128.8. I HURRIEDLY ACKNOWLEDGED. A SMALL CPR JET WAS TAXIING IN FRONT OF US AND TAXIED ACROSS RWY 4L. COPLT MISSED THE HOLD SHORT RWY 4L INSTRUCTIONS AND MISSED FREQ CHANGE; SO I LOOKED DOWN AS I PUT 128.8 IN OTHER HEAD OF VHF #1 RADIO. WHEN I LOOKED UP; I HAD ALREADY PASSED THE RWY MARKINGS FOR RWY 4L AND COULD NOT STOP IN TIME TO HOLD SHORT OF RWY; SO ADDED PWR TO CROSS AS FAST AS POSSIBLE. ONCE PAST RWY 4L; TWR CTLR ON RWY 4L TOLD US TO MONITOR GND CTL. NOTHING FURTHER WAS SAID. FATIGUE WAS A DEFINITE FACTOR. WX IN NE CORRIDOR WAS BAD. RADIO CHATTER AT THE TIME WAS EXCESSIVE ON BOTH APCH AND TWR FREQS. I HAD TO CHK IN A NUMBER OF TIMES ON APCH; AFTER LNDG CLRNC FROM TWR; CHATTER CONTINUED. THIS WAS THIRD DAY OF 4 DAY TRIP WITH MANY WX DELAYS; LATE ARRS; ETC. WE WERE BOTH EXTREMELY TIRED.