>>> import gc
>>> gc.mem_free()
7904
>>> gc.collect()
>>> gc.mem_free()
60464

I guess Micropython can not see those 4M PSRAM part.
What can I do so that I can use the 4M PSRAM in M5Stack Fire?

During the booting process, I saw the following. Does that mean the 4M PSRAM was not initiated?

W (129) MicroPython: SPIRAM: Disabled

I also tried the following and got:

>>> import micropython, machine
>>> micropython.mem_info()
stack: 752 out of 14336
GC: total: 86016, used: 29712, free: 56304
 No. of 1-blocks: 410, 2-blocks: 75, max blk sz: 654, max free sz: 2263
>>> machine.heap_info()
Heap outside of MicroPython heap:
---------------------------------
              Free: 95208
         Allocated: 187096
      Minimum free: 87724
      Total blocks: 200
Largest free block: 93252
  Allocated blocks: 196
       Free blocks: 4

It seems I only have 56k free RAM if I am correct.