Well I'm about 2 years late to this party but as a first year MET student I find this thread unbelievably interesting. I'm currently learning about all the same concepts applied here in my metals lab/class, which makes this even cooler. I still haven't taken thermo (first year student) so if anyone does read this please correct me if I'm wrong, but could this at all correlate to cylinder 4 experiencing repeated excessive heat cycles causing a portion of the piston to "temper" and become softer in just that one region? The excessive heat being caused by the runner design/unevenly flowing injectors after 100k+ miles/garbage OEM tunes; or a combination of them all? Especially if an injector starts to go once the car reaches higher mileage and doesn't flow right. I could be very wrong in saying this but couldn't an uneven flowing injector cause a lean condition in one region of the piston?
Regardless finding this thread made my night. Glad I'm not the only Subaru obsessed engineering student around lol