FLC OF CPR JET CLRED 6000 TO EXPEDITE; WENT INTO 4000 FPM CLB; AT 4200 MSL CLRED TO MAINTAIN 4000.

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DURING DEP FROM CAPITAL ARPT IN SPRINGFIELD; IL; (THE HOME BASE OF THE CREW AND TURBOJET) I WAS THE PF IN THE L SEAT AND I WAS SECOND IN COMMAND. WE WERE CLRED BY SPI DEP CTL TO CLB TO 4000 FT AND TURN TO 080 DEG HDG. AT APPROX 3000 FT MSL DEP TOLD US TO CLB TO 6000 MSL AND EXPEDITE! I IMMEDIATELY COMPLIED WITH MAX PWR AND 10-15 DEG DECK ANGLE. WE WERE CLBING AT 4000-6000 FPM AND PASSING THROUGH 4200 MSL WITH THE HIGH RATE OF CLB REQUESTED BY ATC; WHEN WE WERE TOLD TO MAINTAIN 4000 FT MSL. I RESPONDED IMMEDIATELY BY REDUCING PWR AND DECREASING THE DECK ANGLE AND WE WERE LEVELING AT 5000 MSL WHILE THE PNF WAS TRYING TO DETERMINE FROM ATC IF WE SHOULD CONTINUE TO 6000 OR GO TO 4000; BECAUSE IT BECAME CLR THAT THERE WAS A COMMUTER DSNDING THROUGH 5000 TO 4000 THAT ATC WAS TRYING TO SEPARATE US FROM. AT THIS POINT WE WERE JUST 'BREAKING OUT' OF THE CLOUDS ON TOP AND SAW THE COMMUTER GO INTO THE CLOUDS ABOUT 4 MI FROM OUR POS. WE WERE THEN CLRED TO 6000. THE ONLY ACTION THAT CAN PREVENT THIS TYPE OF OCCURRENCE IS FOR AIR TFC CTLRS TO BE BETTER BRIEFED AND HAVE A BETTER UNDERSTANDING OF HIGH PERFORMANCE TURBOJETS. IF ATC ASKS A TURBOJET TO EXPEDITE TO AN ALT; THEY SHOULD EXPECT ANYTHING FROM 3000-6000 FPM IN CLB PERFORMANCE WHEN AT LOW ALTS SUCH AS 4000 MSL.