CRJ200 FLT CREW HAS A RWY CONFLICT AT LGA.

Narrative

DEPARTING LGA; THERE WAS A FLT XXX ABOUT 5 ACFT IN FRONT OF US. THEY DEPARTED WITH NO PROB. 15 MINS LATER WE WERE ISSUED A CLRNC; 'ACR YXX; POS AND HOLD RWY 4.' WE TAXIED INTO POS. COUPLE MINS LATER WE HEARD; 'ACR XXX CLRED FOR TKOF RWY 4.' WE HESITATED A FEW SECONDS. NO ONE SAID ANYTHING. THEN I RESPONDED; 'ACR YXX CLRED FOR TKOF RWY 4;' EMPHASIZING THE Y IN YXX. WE STILL HESITATED A FEW SECONDS AND NOTHING WAS SAID SO WE DEPARTED. ONE AND A HALF HRS WENT BY AND WE STARTED TO DSND INTO DEST. THEN ATC CAME ON AND SAID FOR US TO CALL LGA TWR UPON LNDG. I DID NOT TALK TO TWR ON THE PHONE; BUT THE CAPT DID. WHAT I WAS TOLD WAS THAT THERE WAS ANOTHER FLT; ACR YZX ON THE APCH AND THEY WERE ISSUED A CLRNC TO LAND ON RWY 31 AND THAT WE TOOK IT AS FOR TO TKOF RWY 4. THERE WERE 3 ACR FLTS WITHIN 15 MINS WITH SIMILAR CALL SIGNS AND I BELIEVE WE OR THE TWR HAD GOTTEN THE NUMBERS CONFUSED. THE TWR TRIED TO LISTEN TO THE TAPES BUT SAID THEY WERE TOO SCRATCHY TO UNDERSTAND WHAT WAS SAID. LOOKING BACK; I BELIEVE WE SHOULD HAVE WAITED UNTIL TWR CONFIRMED THAT HE DID INDEED GIVE US PERMISSION TO TKOF. AT A HIGH VOLUME ARPT LIKE NEW YORK; YOU ARE TRAINED THAT WHEN YOU ARE PUT ON THE RWY; YOU NEED TO BE READY TO GO WHEN TOLD OR YOU LOSE YOUR SLOT AND SLOW DOWN THE REST OF THE LINE. WE ACTED ON WHAT WE BOTH BELIEVE WAS JUST A MIX UP IN NUMBERS BECAUSE THERE WAS A PREVIOUS FLT WITH SIMILAR NUMBERS AND THEY HAD ALREADY TAKEN OFF.