A C172 PLT INADVERTENTLY TAXIES OFF THE TXWY AT NIGHT INTO A DITCH.

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AFTER LNDG ON RWY 21R; WE EXITED THE RWY ONTO TXWY C. GND CLRED US TO TAXI BACK TO PARKING. SHORTLY AFTER GND AMENDED OUR TAXI INSTRUCTIONS; GND TOLD US TO OFFSET AT C4 TO THE N RAMP FOR OPPOSITE TFC. BOTH ME AND THE OPPOSITE TFC ONLY HAD THE NAV LIGHTS AND BEACONS ON. THE OPPOSITE TFC WAS STOPPED ON THE OTHER SIDE OF C4 WAITING FOR ME TO TURN ONTO C4. I WAS NAVING THE TXWY ONLY BY THE TAXI EDGE LIGHTS. I SAW THE SIGN FOR C4. SINCE I THOUGHT THE SIGN FOR C4 WAS ON THE FAR SIDE OF THE TXWY; I PROCEEDED TO TURN IN FRONT OF THE SIGN. THE SIGN FOR C4 WAS ON THE NEAR SIDE AND I ENDED UP TURNING EARLY INTO THE GRASS AND INTO THE DITCH. UPON STOPPING; I SHUT THE ENG DOWN AND CALLED GND TO LET THEM KNOW I WAS IN THE DITCH. THE ONLY VISUAL DAMAGE TO THE ACFT WAS THE PROP WAS SCRATCHED UP ON THE TIPS. TO HELP PREVENT THIS FROM HAPPENING; I RECOMMEND PRC SET A STANDARDIZED DISTANCE FOR ALL TXWY EDGE LIGHTS. THERE WAS NOT SET STANDARD DISTANCE MAKING JUDGEMENT OF WHERE ONE IS MORE DIFFICULT THAN IT SHOULD BE. A SECOND THING TO HELP PREVENT THIS IS BETTER SITUATIONAL AWARENESS WHILE TAXIING AT NIGHT.