Use e_steps[EXTRUDERS] instead of e_steps[4]

Marlin/stepper.cpp

   unsigned char Stepper::old_OCR0A;
   long  Stepper::final_advance = 0,
         Stepper::old_advance = 0,
-        Stepper::e_steps[4],
+        Stepper::e_steps[EXTRUDERS],
         Stepper::advance_rate,
         Stepper::advance;
 #endif
       CBI(TCCR0A, WGM01);
       CBI(TCCR0A, WGM00);
     #endif
-    e_steps[0] = e_steps[1] = e_steps[2] = e_steps[3] = 0;
+    for (uint8_t i = 0; i < EXTRUDERS; i++) e_steps[i] = 0;
     SBI(TIMSK0, OCIE0A);
   #endif //ADVANCE
 

Marlin/stepper.h

     #endif
 
     #if ENABLED(ADVANCE)
-      static long e_steps[4];
+      static long e_steps[EXTRUDERS];
     #endif
 
   private: