CL65 FLT CREW RECEIVES CONFUSING TAXI INSTRUCTIONS FROM ORD GROUND AND TAXIES TO 9L INSTEAD OF 4L. FLT CREW CROSSES HOLD LINE AT 9L BEFORE REALIZING THEIR MISTAKE.

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THE INCIDENT WAS PRIMARILY CAUSED BY FLT CREW COMPLACENCY; IN MY OPINION; AND NOT ENOUGH ATTN PAID ON THE PART OF THE FLT CREW TO ALL ATC XMISSIONS. SITUATION WOULD HAVE BEEN AVERTED IF WE HAD HEARD AND UNDERSTOOD THAT THE MD-80 WAS CLRED FOR TKOF FROM RWY 9L. IT ALSO WOULD HAVE HELPED SIGNIFICANTLY IF THE GND CTLR HAD TOLD US TO 'PASS BEHIND THE MD80' INSTEAD OF 'FOLLOW THE MD80;' OR IF HE HAD MENTIONED TO US THAT THE MD80 WAS ON HIS WAY TO RWY 9L. TAXI CLRNC WAS 'BRAVO-JULIET-WHISKEY TO RWY 4L; FOLLOW THE MD80 FROM L TO R.' WE WAITED FOR THE MD80 TO PASS AND THEN FOLLOWED IT UP BRAVO AND JULIET. IT WAS A RELATIVELY SHORT TAXI RTE; AND THERE WAS NO LINE-UP FOR DEP; WHICH WAS UNUSUAL. WE HAD TAXIED OUT SINGLE-ENG; SO WE WERE SCRAMBLING A LITTLE TO GET THE OTHER ENG STARTED AND RUN THE CHKLISTS. WE KNEW WE WERE TAXIING TO RWY 4L; BUT WE DID NOT REALIZE THAT THE MD80 WAS TAXIING TO RWY 9L. I WAS PREOCCUPIED AND TOO RELAXED; AND SIMPLY FOLLOWED THE MD80 TO RWY 9L. WE DID NOT HAVE TIME TO THINK THINGS OVER; BECAUSE THE MD80 WAS IMMEDIATELY CLRED FOR TKOF ON RWY 9L; AND SECONDS LATER WE WERE CLRED INTO POS AND HOLD RWY 4L. WE ASSUMED WE WERE BEING CLRED INTO POS BEHIND THE MD80. I CLRED FINAL AS WE TAXIED OUT; AND I WAS CERTAIN THERE WAS NO ONE ON FINAL. AS WE BEGAN TO TAXI OUT; THE PICTURE BEGAN TO SEEM WRONG. I BRAKED TO A STOP WHILE STILL ON THE PORTION OF THE TXWY PERPENDICULAR TO THE RWY; BUT OVER THE HOLD SHORT LINE. AT THIS POINT TWR CALLED AND ASKED US IF WE WERE TAXIING ONTO RWY 9L. I REPLIED THAT WE WERE. TWR SAID 'OK; JUST CONTINUE INTO POS AND HOLD RWY 9L.' WE DID SO; AND HELD FOR ABOUT 20 SECONDS ON THE RWY BEFORE BEING CLRED FOR TKOF. IT WAS CLR THAT THERE WAS NO TFC CONFLICT; AND TWR HAD NO FURTHER COMMENTS TO US ABOUT THE SITUATION. I BRAKED TO A STOP UPON REALIZING THAT THE RWY ENVIRONMENT DID NOT LOOK RIGHT. TWR ALSO MADE A CALL TO US AT ABOUT THIS SAME TIME. AS STATED ABOVE; I THINK GND CTL SHOULD BE CAREFUL ABOUT ADVISING ACFT TO 'FOLLOW' OTHER ACFT THAT ARE TAXIING TO DIFFERENT RWYS. HOWEVER; I AM AWARE OF THIS PRACTICE; AND I SHOULD HAVE CAUGHT IT.