Org-mode source code blocks indent when I RET

Troubleshooting
4

The docstring of org-indent-line has the following:

  - In the code part of a source block, use language major mode
    to indent current line if ‘org-src-tab-acts-natively’ is
    non-nil.  If it is nil, do nothing.

I suggest you check the value of org-src-tab-acts-natively; it’s probably t for you. Then I think it’s the matter of the language major mode you use for the src block.

My guess is that it’s set to 4 spaces for the major mode, which gets added to org-edit-src-content-indentation as @akashp mentions above – my pure guess. I don’t use src block in Org mode extensively.