RWY INCURSION AS AN MD80 TAXIES PAST TXWY P ONTO RWY 4 AT BWI; MD.

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WE WERE CLRED TO TAXI TO RWY 15R AT BWI ARPT FROM OUR GATE. THE TAXI RTE ASSIGNED WAS TXWY C; TXWY P; TO RWY 15R. UPON EXITING THE ALLEYWAY; THE CAPT CALLED FOR THE TAXI CHKLIST. AS I WAS RUNNING THE CHKLIST; I LOOKED UP AS THE GND CTLR STATED WE WERE ENTERING RWY 4 AT TXWY C. HE SAID NO PROB; TAXI DOWN RWY R TO TXWY P1; THEN TXWY P; TXWY A TO RWY 15R. SO; WE CONTINUED THE TURN ONTO RWY 4 AND FOLLOWED HIS DIRECTIONS. AT THE TIME; ATIS WAS CALLING RWYS 10; 15L; AND 15R IN USE. IMMEDIATELY AFTER US; A B737 ALSO MADE THE R TURN ONTO RWY 4 AT TXWY C. HE WAS ALSO TOLD NO PROB AND GIVEN THE SAME TAXI INSTRUCTIONS WE WERE. BECAUSE THE RWY WAS NOT IN USE; NO CONFLICT WITH ANOTHER ACFT WAS INVOLVED. CONTRIBUTING TO THIS OCCURRENCE WERE SEVERAL FACTORS: RUNNING THE CHKLIST DISTR ME AND THE CAPT WAS CONFUSED BY THE TAXI DIAGRAM. INTENDING TO MAKE THE FIRST TURN OUT OF THE ALLEYWAY; HE TURNED PAST THE FIRST ISLAND HE SAW; WHICH WAS ACTUALLY THE ISLAND BTWN TXWY P AND RWY 4. ALSO; THIS INTXN IS NOT LABELED AS ONE OF THE HOT SPOTS AT BWI. SUPPLEMENTAL INFO FROM ACN 557733: I CONSULTED THE ARPT DIAGRAM AND REVIEWED THE RTE. THAT TURN FROM TXWY C IS STAGGERED LATERALLY FROM THE TURN ONTO RWY 4. I DIDN'T SEE THE TURN ONTO TXWY P AND MADE THE FIRST TURN THAT I SAW.