B737-300 DEPARTING FROM RWY 4L AT ORD INITIATED ABORT AFTER TWR INSTRUCTIONS WERE CONFUSING; OBSERVING A CLOSELY SPACED GAR ON RWY 9L.

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WE WERE CLRED INTO POS ON RWY 4L. WHILE STILL MOVING INTO POS; WE RECEIVED TKOF CLRNC; AND WERE TOLD THAT TFC IS '1.5 MI OUT.' WE PUSHED THE PWR UP AND HAD JUST STARTED TO ROLL WHEN THE TWR CALLED FOR AN ACR Y (DON'T REMEMBER THE WHOLE CALL SIGN) ABORT TKOF. THAT ACFT CAME BACK AND SAID THAT THEY WERE ALREADY AIRBORNE. TWR THEN AGAIN SAID TO ABORT TKOF (NO CALL SIGN GIVEN). I CALLED BACK AND ASKED QUICKLY IF THAT WAS FOR ACR X. ONLY REPLY WAS 'ABORT TKOF;' FOLLOWED QUICKLY BY THE TWR CONTINUING; '(CALL SIGN) GAR.' BECAUSE OF THE CONFUSION I IMMEDIATELY ABORTED; NO MORE THAN 30 KTS. I HAD NOT CROSSED RWY 9L YET; AND LOOKED UP AND SAW AN RJ ON THE GO NO MORE THAN 200 FT HIGH. TWR TOLD US TO TURN R ON RWY 9L; R AGAIN ONTO TXWY B BACK TO TXWY E FOR RWY 4L AGAIN. WE WERE CLRED INTO POS AND A NON EVENTFUL TKOF. TWR SEEMED IN A RUSH TO GET ACFT OFF THE GND. THAT LEFT US WITH LITTLE SPACING ON LNDG ACFT. I WAS UNABLE TO SEE THE ARRIVING ACFT PRIOR TO TAKING THE RWY. IF I HAD NOT ABORTED WHEN I DID WE WOULD HAVE ENTERED RWY 9L AND THE ACFT GOING AROUND WOULD HAVE FLOWN OVER US PROBABLY LESS THAN 200-300 FT.