Yes, the heap is for dynamic allocation.
So you simply have to add heap_4.c to your build and define a suitable heap size (used by the heap_4.c implementation) in FreeRTOSConfig.h as mentioned as number and NOT the source file name as you did.
That’s wrong:
#define configTOTAL_HEAP_SIZE 'heap_4.c'
That’s right (as example):
#define configTOTAL_HEAP_SIZE 65536