Lumber is regular american 2X4s.
American 2x4’s haven’t been 2”x4” in a long time
No kidding. I recently renovated the bathroom in my old shotgun shack, which involved tearing out part of a wall, and replacing it.
The inexpensive method of addressing that issue is to rip modern wood to provide tack strips which bring the replacement lumber back to 4". Air powered staple guns are heaven sent.
Try tearing out even somewhat rotted oak real 2x4s. My brother did mention that 6 lb sledge hammers weren't typical framing tools.
/johnny